Sedimentary Geochemistry Research Papers - Academia.edu (original) (raw)

Umhlatuzana rockshelter has an occupation sequence spanning the last 70,000 years. It is one of the few sites with deposits covering the Middle to Later Stone Age transition (~40,000-30,000 years BP) in southern Africa. Comprehending the site's depositional history and occupation sequence is thus important for the broader understanding of the development of Homo sapiens' behavior. The rockshelter was first excavated in the 1980s by Jonathan Kaplan. He suggested that the integrity of the late Middle Stone Age and Later Stone Age sediments was compromised by large-scale sediment movement. In 2018, we initiated a high-resolution geoarchaeological study of the site to clarify the site formation processes. Here, we present the results of the excavation and propose a revised stratigraphic division of the Pleistocene sequence based on field observations , sedimentological (particle size) analyses, and cluster analysis. The taphonomy of the site is assessed through phytolith and geo...

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- Archaeology, Stable Isotope Analysis, Geoarchaeology, Archaeological Stratigraphy

Sediment geochemistry is a powerful tool to investigate earth surface processes because sediments record geochemical signatures of their provenance, generation, transport, deposition, and environmental conditions. A significant advancement in the technology of analytical geochemistry has made geochemical analyses of sediments more rapid and precise, which together provided an impetus to the study of sediment system. As a result, high-quality research papers on sediment geochemistry have been published during the last five years from India. A foreseeable growth of research and an increase in the publication are likely in the future because of the new setups of state-of the-art analytical instruments for geochemical research at various research and academic institutions in India. Here we are presenting the published important work on sediment geochemistry under three categories, i.e., weathering, erosion, deposition and provenance determination; sediment geochemistry as palaeoclimate proxy; and environmental geochemistry.

- by Md Maroof Azam
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- Provenance, Palaeoclimate, Sedimentary Geochemistry, Weathering

Dedicated to Professor Dr Jochen Hoefs on the occasion of his 75th birthday Oxygen isotope fractionations in double carbonates of different crystal structures were calculated by the increment method. Synthesis experiments were performed at 60 °C and 100 °C to determine oxygen and carbon isotope fractionations involving PbMg[CO 3 ] 2. The calculations suggest that the double carbon-ates of calcite structure are systematically enriched in 18 O relative to those of aragonite and mixture structures. Internally consistent oxygen isotope fractionation factors are obtained for these minerals with respect to quartz, calcite and water at a temperature range of 0–1200 °C. The calculated fractionation factors for double carbonate–water systems are generally consistent with the data available from laboratory experiments. The experimentally determined fractionation factors for PbMg[CO 3 ] 2 , BaMg[CO 3 ] 2 and CaMg[CO 3 ] 2 against H 2 O not only fall between fractionation factors involving pure carbonate end-members but are also close to the calculated fractionation factors. In contrast, experimentally determined carbon isotope fractionation factors between PbMg[CO 3 ] 2 and CO 2 are much closer to theoretical predictions for the cerussite–CO 2 system than for the magnesite–CO 2 system, similar to the fractionation behavior for BaMg[CO 3 ] 2. Therefore, the combined theoretical and experimental results provide insights into the effects of crystal structure and exchange kinetics on oxygen isotope partitioning in double carbonates.

- by Yong-Fei Zheng
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- Stable Isotope Geochemistry, Sedimentary Geochemistry, Carbonates
- by Jia-Zhong Zhang
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- Biogeochemistry, Sedimentary Geochemistry

Aluminum (Al) treatment is one of the most commonly used approaches to reduce internal phosphorus (P) loading in lakes. However, the adequate amount of Al that should be added to permanently inactivate mobile (releasable) sediment P can be determined using many different methods. These methods differ substantially in their specified design sediment depth, targeted P pool(s), and expected binding ratio. In this study, Al doses for inactivating sediment P in Beung Gii Lake of Thailand were determined using the most commonly used methods reported in literature and then compared. Experimental procedures included sediment P fractionation, Al assay experiments, and a geochemical model. Mobile P was detected in the lake’s sediment at 2.52, 5.42, and 7.65 g/m2 in the upper 4, 10, and 15 cm, respectively, with additional P contained in labile organic form. Comparing the resulting Al doses for the lake, it was found they varied by nearly an order of magnitude (45–306 g Al/m2). This result hig...

- by Anthony Kuster
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- Chemistry, Environmental Management, Sedimentary Geochemistry, Medicine

The sources of the stone used to construct Stonehenge around 2500 BCE have been debated for over four centuries. The smaller “bluestones” near the center of the monument have been traced to Wales, but the origins of the sarsen (silcrete) megaliths that form the primary architecture of Stonehenge remain unknown. Here, we use geochemical data to show that 50 of the 52 sarsens at the monument share a consistent chemistry and, by inference, originated from a common source area. We then compare the geochemical signature of a core extracted from Stone 58 at Stonehenge with equivalent data for sarsens from across southern Britain. From this, we identify West Woods, Wiltshire, 25 km north of Stonehenge, as the most probable source area for the majority of sarsens at the monument.

- by David Nash and +1
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- Archaeology, Geochemistry, Principal Component Analysis, Neolithic Archaeology

Climate change, a timely topic, cannot be understood solely by analyzing modern-day ecosystems. Sediment stratigraphy from tidal marshes is an important source of paleoecological data, as these ecosystems experience high rates of deposition and preserve organic material well. Although Hudson River Valley marshes have been extensively researched, work has focused on the four areas protected by the Hudson
River National Estuarine Research Reserve (HRNERR). The marshlands of Haverstraw in Rockland County are comparable to the HRNERR sites in their capacity for biodiversity, carbon sequestration, and other important ecological functions. They are, however, understudied. This study uses loss-on-ignition and macrofossil analysis in combination with X-ray fluorescence spectroscopy to construct a high-resolution
paleoenvironmental record. Biotic and geochemical zones have been identified and correlated with archaeological and historical data to assess the influence of anthropogenic activity in the area. The organic:inorganic maxima evidenced likely correspond to a preindustrial period, characterized by burning events associated with increasing populations and concentrated settlement, as well as agricultural practices. Invasive species are prevalent, but certain key native species, notably Acorus americanus (American Sweet Flag), persist today. Exponential increases in heavy metal concentrations likely result from industry in the area but have declined following cessation of this activity and introduction of unleaded gasoline. Understanding historical human impacts is vital for predicting the ramifications of future development as well as establishing protocols for conservation and restoration.

- by Lucy Gill
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- Paleoenvironment, Paleoecology, Invasive species ecology, Wetland Ecology

The petrographic and geochemical composition of the Dharla River sediments has been examined to infer their sediment type, degree of weathering, provenance, and tectonic settings. Petrographically the sediments are characterized by the high quartz content (64.97 to 74.24 wt%), followed by feldspar (7.04 to 15.20 wt%), mica (5.38 to 19.92 wt%), lithic fragment (3.46 to 8.14 wt%), and heavy minerals (1.98 to 6.94 wt%). Geochemical composition shows marked enrichment of SiO 2 (mean ~ 74.16%) and a strong negative correlation with the other major oxides because of quartz dilution. The Chemical index of alteration (CIA, 45.52 to 63.51); Plagioclase index of alteration (PIA, 43.13 to 66.55); W index (20.15 to 32.86) and Rb/Sr ratios (0.35 to 0.98) suggest low to moderate intensity of chemical weathering in the source area. Geochemical classifications of the studied samples show mostly litharenitic immature type of sediments also reflects high index of compositional variability (ICV, 0.96 to 1.72). The ternary diagrams of Al 2 O 3-(CaO + Na 2 O)-K 2 O (or A-CN-K) and of mafic rocks, felsic rocks and degree of weathering of the source rocks (or MFW) and several immobile trace element ratios (e.g. light rare earth element/heavy rare earth element or LREE

- by Md Mahabubur Rahman
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- Geology, Geochemistry, Mineralogy, Trace element Geochemistry

Detrital sand grains from three beaches (Tecolutla,
Nautla, and Veracruz) along the western Gulf of Mexico were
studied by a scanning electron microscope, to investigate the
depositional environment and paleoclimate. Totally, 24
microtextures are identified; among them, 13 are grouped as
mechanical origin, 5 as mechanical and/or chemical origin, and
6 as chemical origin. These microtextures are nonuniformly
distributed among the three beach areas. Concoidal fractures,
straight and arcuate steps at Tecolutla and Veracruz beaches
indicate that the sand grains were derived from the crystalline
rocks. The abundance of angular outline grains at the Nautla
beach supports for short transportation probably close to the
source area. The domination of rounded sand grains in the
Veracruz beach reveals that the sediments were derived by
the aeolian mechanism. Chattermark trials at the Veracruz
beach sands are indicating a wet tropical climate. Chemical
features like silica globules, silica pellicle, and trapped diatoms
in the Tecolutla and Veracruz beach sands suggest a silica
saturated environment. Similarly, chemical etching and solution
pits are common in the Veracruz beach sands, which are
probably linked to the contaminated sea water. Desiccation
crack at Veracruz beach sands is an indicator of temperature
changes in the beach environment. Broken benthonic foraminifera Elphidium discoidale sp. present in the Veracruz beach indicates a high-energy littoral environment.

The weathering conditions, provenance, and tectonic setting of sands from the San Nicolas (SN) and San Carlos (SC) beaches along the Gulf of California, Mexico, have been studied using mineralogy, major element, and trace element data. The low positive Eu anomaly in the SC sands was probably due to the contribution of sediments from intermediate rocks between felsic and mafic compositions. The comparison of rare earth element data of the sands with rocks located relatively close to the study areas revealed that the SN sands received a major contribution from felsic rocks and SC sands from intermediate rocks. The compositional difference between the SN and SC beach areas indicated that longshore currents played a less significant role. Discriminant function-based major element diagrams for the tectonic discrimination of siliciclastic sediments revealed a rift setting for the Gulf of California, which is consistent with the general geology of Mexico.

This article deals with some geochemical parameters of some selected chemical data of the younger granite of Egypt. The younger granites have a composition ranging from Alkali feldspar granite to syenogranites. the major oxides (FeO, CaO, MgO, Al2O3, MnO, Na2O) show symmetrical variation which indicated continuous variation and fractional crystallization from the pure magmatic melt. It was concluded that these rocks exhibit alkaline affinity and fractional crystallization from the magmatic melt. This younger granites are classified according to their geological setting and petrography to belong to Phase III which is mainly alkali granites. The younger granite of Egypt seems to be homogeneous chemically and mineralogically, unfoliated, hard resist weathering to a great extent. The relations between Al2O3 and SiO2 indicate that most investigated samples are post-orogenic granite(POG). These results are also confirmed by using the relation between (FeOt/FeOt+MgO) and SiO2 wt%.

The geochemistry of stream sediments deals with the study or the comparison of major and trace element concentrations of an area using the stream sediments of the studied area. Stream sediments contain fragments that are derived from the erosion of weathered rocks by stream or river water. Depending on their particle size and stream water velocity, they are transported in suspension, by saltation or by creeping along the stream bed. Thus, they represent the source catchment area of the stream drainage network. Collected stream sediment samples can be analyzed and used to study various geochemical factors such as provenance studies and detection of mineral resources for mining.
Since stream sediments usually give an indication of all the minerals available upstream, they are usually used for low-cost reconnaissance survey for mining explorations.
A case study of a stream sediment geochemical survey of the Ganga River headwaters in the Garhwal Himalaya by Mukherjee et al, (2007) was considered. From the review, it can be concluded that stream sediments are indeed useful in the study of the distribution of mineable materials and also for the production of geochemical maps.

- by Victor Towoju
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- Geology, Geochemistry, Sedimentary Geochemistry, Stream Sediments

This article talks about fluorite and barite mineralization in a sector of western Sicily near the Tyrrhenian Sea. These mineralization in a small area, the Poggio Balate hill, north-west of Monte San Calogero Termini Imerese (Sicily) take on the characteristics of a real, intensely mineralized deposit. It is a low-grade hydrothermal deposit involving well-cemented Mesozoic clastic limestone rocks. The discovery occurred during the search for a stratigraphic geology work in the early 1970s. The fluorite and barite mineral deposit of Poggio Balate is the only one existing in all of Sicily. Some geological aspects covered make this publication still relevant in light of the results of recent geophysical studies on geothermal resources within the carbonate reservoirs of western Sicily (Italy). The geochemical phenomenon that produced this surprising mineralization is related to a hydrothermal circulation that ascended along the fault system in rocks, predominantly Mesozoic carbonate rocks.

- by Francesco Speciale
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- Geophysics, Sedimentary Geochemistry, Active Tectonics, Tectonic Geomorphology

Los sedimentos de la cuenca del Usumacinta son compuestos de arcilla, limo, barro y grava, esenciales por albergar una vida secreta y participar en la dinámica de este ecosistema fluvial. Los lugareños los llaman “arena”, “guijarros” o “piedras”, y han fungido como recursos desde tiempos inmemoriales para la producción de cerámica, la mejora del suelo o como material de construcción. Las sociedades se han expandido y alimentado gracias a ellos, pero con el crecimiento urbano y la transformación de los ecosistemas, hemos alterado sus dinámicas de generación, transporte y acumulación; ni más ni menos que todo aquello que resulta fundamental para la vida de un territorio conformado por una cuenca y un río compartido por Tabasco, Chiapas y el altiplano guatemalteco.
Con esta publicación buscamos explicar en doce preguntas la naturaleza e importancia de los sedimentos del Usumacinta: ¿para qué se utilizan actualmente?, ¿qué usos han tenido a lo largo del tiempo?, ¿cómo pueden aprovecharse? ¿están contaminados? Nuestras respuestas buscan contribuir a la construcción de una relación sostenible entre el Usumacinta y los seres humanos que viven en sus riberas. Con este ejemplo bien podría abordarse la misma problemática respecto a cualquier otra cuenca.
Esta publicación se realizó en el marco de una estrecha colaboración científica con la participación de investigadores mexicanos y franceses que, iniciada en 2018, concluye en 2022: el proyecto VAL-USES
“Desde el aprovechamiento tradicional hacia la valorización integral de los sedimentos en la cuenca del río Usumacinta”, un proyecto interdisciplinario aprobado y financiado por la Agencia Nacional de Investigación en Francia y el Consejo Nacional de Ciencia y Tecnología en México.

The deposits of large Asian rivers with unique drainage geometries have attracted considerable attention due to their explanatory power concerning tectonism, surface uplift and upstream drainage evolution. This study presents the first petrographic, heavy mineral, Nd and Sr isotope geochemistry, and detrital zircon geochronology results from the Holocene Irrawaddy megadelta alongside modern and ancient sedimentary provenance datasets to assess the late Neogene evolution of the Irrawaddy River. Contrary to models advocating a steady post-middle Miocene river, we reveal an evolution of the Irrawaddy River more compatible with regional evidence for kinematic reorganization in Myanmar during late-stage India-Asia collision. Quaternary sediments are remarkably consistent in terms of provenance but highlight significant decoupling amongst fine and coarse fraction 87 Sr/ 86 Sr and ε Nd due to hydraulic sorting. Only well after the late Miocene do petrographic, heavy mineral, isotope geochemistry, and detrital zircon U-Pb results from the trunk Irrawaddy and its tributaries achieve modern-day signatures. The primary driver giving rise to the geometry and provenance signature of the modern Irrawaddy River was regional late Miocene (≤10 Ma) basin inversion coupled with uplift and cumulative displacement along the Sagaing Fault. Middle to late Miocene provenance signatures cannot be reconciled with modern river geometries, and thus require significant loss of headwaters feeding the Chindwin subbasin after ∼14 Ma and the northern Shwebo subbasin after ∼11 Ma. Large-scale reworking after ∼7 Ma is evidenced by modern Irrawaddy River provenance, by entrenchment of the nascent drainage through Plio-Pleistocene inversion structures, and in the transfer of significant sediment volumes to the Andaman Sea.

Compositional and geochemical analyses of sands collected from the Chachalacas (CHA) and Veracruz (VER) beach areas along the western Gulf of Mexico were studied to determine the provenance and tectonic setting of the source region. The modal composition showed that the proportion of quartz (Q) is
lower in CHA than in VER sands. The average quartz-feldspar-lithic fragment (QFL) ratios for the CHA and VER sands are Q75F8L17 and Q86F4L10, respectively. The X-ray diffractometer (XRD) and Scanning Electron
Microscope equipped with EDAX spectrometer (SEM-EDS) data revealed that the CHA sands were abundant in heavy minerals like magnetite, ilmenite, and zircon. The rare earth element concentration (REE) is higher in CHA than in VER sands, which is due to the concentration of heavy minerals in CHA
sands. The weathering indices such as chemical index of alteration (CIA), plagioclase index of alteration,
and AeCNeK (A ¼ Al2O3, CN¼CaO* þ Na2O, K ¼ K2O) plot suggested that the intensity of weathering in
the source area was low to moderate. The index of chemical variability (ICV) for the CHA (~1.9e3.0) and
VER (~0.82e1.33) sands indicated that the compositional maturity was higher for the VER sands. The concentrations of Co, Cr, Ni, and V are lower in VER sands than in CHA sands, indicating that the CHA sands were derived from the intermediate source rocks. Provenance modelling revealed that the CHA sands were associated with the mixture of basalt, andesite, dacite, and trachyandesite in the ratio of
5:20:25:50. The VER sands were best matched with a mixture having 75e90% dacite and 25e10% andesite compositions. The provenance difference between the two beach areas suggested that longshore current play a less significant role in mixing and homogenization of sands. The multidimensional tectonic discrimination diagrams revealed rift and collision settings for the VER and CHA beach areas, respectively, which is consistent with the general geology of the study areas.

- by John S. Armstrong-Altrin
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- Geochemistry, Sedimentology, Tectonics, Sedimentary provenance

Sulas, F., Bagge, M.S., Enevold, R., Harrault, L., Kristiansen, S.M., Ljungberg, T., Milek, K., Mikkelsen, P.H., Jensen, P.M., Orfanou, V., Out, W.A., Portillo, M. Sindbæk, S.M. 2022. Revealing the invisible dead: integrated... more

- by Federica Sulas and +2
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- Archaeology, Biomarkers, Archaeological Soil Micromorphology, Palynology

Middle Proterozoic Gulcheru Quartzite marks the onset of sedimentation, after the Eparchaean unconformity, in the Cuddapah Basin. In the southwestern margin of the basin it is made up dominantly of orthoquartzites. On the
basis of lithology, stratigraphic relationship, bedding characteristics, texture and colour of sediments and sedimentary structures, five lithofacies were recognized within Gulcheru Quartzite. The Gulcheru Quartzite, though dominantly of shallow marine origin, shows imprints of other depositional regimes. Detailed interpretation of the facies and study of their field relationships reveal that pink massive quartzite, the lowermost unit of Gulcheru Quartzite, is of fluvio-aeolian origin. Later marine transgression led to the development of a moderate to low energy beach, which evolved with time
into a barrier-spit complex.

- by Himadri Basu
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- Sedimentology, Sedimentary Geochemistry

Southeastern part of Vindhyan–Mahakoshal basin developed along Son–Narmada mega rift zone exposes Palaeoproterozoic Mahakoshal and Meso- to Neo-proterozoic Vindhyan Supergroup of rocks over Chhotanagpur Granite Gneissic Complex (CGGC). Both Mahakoshals and Vindhyans are affected by nearly 30 km long and 1 to 3 km wide ENE-WSW trending fault/shear zone, sympathetic to Son–Narmada mega lineament, extending from Tarka in the east to Baharia in the west in parts of Sidhi district, M.P. A number of surface radioactivity anomalies (22–540 ppmU3O8, <5 ppm ThO2; n=17) have been recorded in ferruginous breccia, phyllite and limestone in the vicinity of this fault/shear zone, where higher water radon content (14–9690 radon counts/50 sec.; n=156) also forms three anomalous clusters. The lithogeochemical sampling (n=162) in grid pattern across this fault/shear zone has indicated wide variation in trace element contents in different lithounits viz., V (5–873 ppm), Cr (22–3625 ppm), Co (6–246 ppm), Ni (5–702 ppm), Cu (<5–1548 ppm), Zn (<5–2201 ppm), Nb (<5–338ppm) and Zr (<5–4030ppm). Anomalous trace element values are mainly recorded in ferruginous breccia and phyllites/carbon phyllites. Integration of geological, geochemical and water radon data sets have indicated that anomalous trace element and radon concentrations are following the shear/breccia zone trends and forming two parallel bands. Presence of favourable factors such as highly fertile Neoarchaean to Palaeoproterozoic basement-cum-provenance, episodic mafic and acid magmatism and basement reactivation generated channel-ways for hydrothermal activity have ultimately led to remobilisation and concentration of uranium and other trace elements at suitable locales viz. fracture/shear zones.

The geochemistry of sandstones of Awi Formation have been examined in order to infer paleo weathering, provenance and tectonics settings. 10 samples were collected and subjected to whole rock geochemistry (major oxides, trace and rare earth elements). The results show large variation in the major oxides of the sandstones from Awi formation. High SiO2 and Al2O3 and low values for Fe2O3, TiO2 and MgO compared with the Post Archean Australian Shale (PAAS) values. Relatively high Chemical Index of Alteration values (CIA, 63.78 – 91.77; mean: 83.70), Chemical Index of Weathering (CIW, 81.78 – 99.69; mean:93.88), Mineralogical Index of Alteration (MIA, 27.56 – 83.53; mean:67.40) and Index of Compositional Variation (ICV, 0.21 – 1.17; mean 0.53) suggesting intensive weathering at the source area. There is positive correlation between Al2O3 and K2O and Na2O suggesting abundant clay mineral contribution to major oxides during weathering process. Enriched High field strength elements (Zr:225.71; Nb:11.62; Hf:6.15; Th: 16.81 and U:3.48) and bivariate plots of TiO2 against Ni reflects contributions from more felsic provenance for the sediments, with few contributions from mafic source. Tectonic setting discrimination based upon major oxides and rare earth elements suggest deposition in a passive continental margin setting. The characteristics of these sediments reflects fluvial conditions. The geochemical characteristics of the sediments strongly suggest deposition in a humid, non-marine environment.

Sedimentary rocks occurred throughout much of the Late Jurassic Cordil-leran margin of Laurasia. Their tectonic setting and provenance are critical to understanding the evolution of the Cordilleran margin during this time. We review published detrital zircon ages and new and published whole-rock geochemistry of the Peshastin Formation and Darrington Phyllite, Cascade Mountains, Washing-ton State, with the goal of better understanding the tectonic development of the Cordillera and strengthening regional correlations of these sedimentary units. The Peshastin Formation conformably overlies the ca. 161 Ma Ingalls ophiolite complex. Published dating of detrital zircons from a Peshastin Formation sandstone provided a youngest U-Pb age distribution of ca. 152 Ma and a signifi cant U-Pb age distribution of ca. 232 Ma. The Darrington Phyllite is structurally above the Shuksan Greenschist; however, this unit also occurs interbedded with the Shuksan Greenschist. The Darrington Phyllite and Shuksan Greenschist have been grouped into the Easton Metamorphic Suite. Published detrital zircons from a Darrington Phyllite metasandstone have a youngest U-Pb age distribution of ca. 155 Ma and a signifi cant U-Pb age distribution of ca. 238 Ma. New major-and trace-element geochemistry and previously published sandstone petrography suggest that these units were derived from Late Jurassic volcanic arc sources that were predominantly transitional between mafi c and intermediate compositions. Middle to Late Triassic detrital zircon ages and detrital modes suggest that some recycling of older accreted arc terranes also contributed to these sediments; however, this Middle to Late Triassic component could also be fi rst cycle. These units consistently plot on geochemical diagrams in fi elds defi ned by modern back-arc basin turbidites. The youngest detrital zircon age distributions, detrital sandstone petro-graphy, and geochemistry of these units suggest they formed in Late Jurassic arc-fed MacDonald, J.H., Jr., and Dragovich, J.D., 2015, Sedimentary geochemistry of the Peshastin Formation and Darrington Phyllite, Cascade Mountains, Wash-ington State: Provenance, tectonic setting, and regional implications, in Anderson, T.

- by James Macdonald
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- Sedimentary Geochemistry, Jurassic

The Paleoproterozoic (∼2.15 Ga) Franceville Basin, South Eastern Gabon, is a continental sedimentarybasin that host unmetamorphosed sediments. This study involve detailed mineralogy, sedimentology,and petrography of the basal sedimentary units of FA and lower FB (FB1) Formations, from the basinmargin to centre in relation to mineral paragenesis and fluid flow. The FA Formation conglomerate, sand-stone, and mudstone consists lithofacies of mixed fluvial and fluvio-deltaic transitional origin, while theoverlying FB1 Formation includes alternating organic rich black shale and sandstone of marine deposit.The medium- to coarse grained fluvio-deltaic quartz arenite in upper part of FA Formation is charac-terized by pervasive authigenic quartz cementation that reduced the porosity and permeability in theearly stage of burial history. This provides a resistant framework for subsequent diagenetic modificationand also inhibits fluid flow during burial diagenesis. In contrast, the clay and unstable detrital grainsrich fluvial arkosic to sub-arkosic sandstones that escaped early quartz cementation show considerablepressure solution at grain contacts. These arkosics were less porous and permeable when depositedbut transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis andsubsequent precipitation of authigenic mineral cements in the resulting secondary porosities.From the proximal to distal basin and within sample suites, there is no considerable chemical variationin the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipita-tion from a near equilibrium homogenous pore-fluid. The predominance of 1Mtillite polytype in mostlithologies reflects precipitation of the clay minerals and probably other cements in an environmentwith high fluid/rock ratio. Dolomite, anhydrite, barite, and Fe-oxides are the main crystallized authigenicminerals aside illite and Fe-rich chlorite clay minerals. The mineralogical assemblages and textural occur-rences of the rocks suggest that diagenesis and fluid flow in the FA Formation in the Franceville Basin aremainly controlled by depositional facies.

- by Bankole Olabode and +1
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- Sedimentology, Stable Isotope Geochemistry, Sedimentary Petrography, Diagenesis

The late Miocene is a time of strong environmental change in SW Asia. Himalayan foreland stable isotope data show a shift in the dominant vegetation of the flood plains away from trees and shrubs towards more C4 grasslands at a time when oceanic upwelling increased along the Oman margin. We present integrated geochemical and colour spectral records from International Ocean Discovery Program Site U1456 in the eastern Arabian Sea to reconstruct changing chemical weathering and erosion, as well as relative humidity during this climatic transition. Increasing hematite/goethite ratios derived from spectral data are consistent with long-term drying after c. 7.7 Ma. Times of dry conditions are largely associated with weaker chemical alteration measured by K/Rb and reduced coarse clastic flux, constrained by Si/Al and Zr/Al. A temporary phase of increased humidity from 6.3 to 5.95 Ma shows a reversal to stronger weathering and erosion. Wetter conditions can result in both more and less alteration due to the nonlinear relationship between weathering rates, precipitation and sediment transport times. Trends in relative aridity do not follow existing palaeoceanographic records and are not apparently linked to changes in Tibetan or Himalayan elevation, but more closely correlate with global cooling. An apparent opposing trend in the humidity evolution in the Indus compared to southern China, as tracked by spectrally estimated hematite/goethite, likely reflects differences in the topography in the Indus compared to the Pearl River drainage basins, as well as the generally wetter climate in southern China.

- by Peter Clift and +2
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- Marine Geology, Sedimentary Geochemistry, Monsoon, Geology of Indian Ocean

Приводятся результаты детального геологического и минералого-геохимческого исследования ме- таморфических пород, получивших развитие в подошве офиолитового комплекса п-ова Камчат- ский Мыс, Восточная Камчатка. Выявлены несколько минеральных типов метабазитов (от наибо- лее высокометаморфизованных клинопироксен-гранат-амфиболовых пород до альбит-эпидотовых амфиболитов), фиксирующих регрессивный этап изменения исходного относительно высокобари- ческого парагенезиса. Петрогеохимические характеристики исследованных пород позволяют отно- сить их к метаморфизованным производным плутонических пород N-MORB типа (предположитель- но габброидов с толеитовой спецификой). Образование метаморфитов связывается с субдукцией в позднемеловое-палеоценовое время океанической плиты, с формированием инвертированного термального градиента в верхней части субдуцируемой плиты и метаморфизмом базитов в условиях фации гранатовых амфиболитов, последующим их аплифтом и эксгумацией при обдукции надсуб- дукционных офиолитов в эоцене-миоцене.

- by Kirill Krylov
- •
- Tectonics, Sedimentary provenance, Sedimentary Geochemistry, Plate Tectonics

Atmospheric CO 2 and global climate are closely coupled. Since 800 ka CO 2 concentrations have been up to 50% higher during interglacial compared to glacial periods. Because of its dependence on temperature, humidity, and erosion rates, chemical weathering of exposed silicate minerals was suggested to have dampened these cyclic variations of atmospheric composition. Cooler and drier conditions and lower non-glacial erosion rates suppressed in situ chemical weathering rates during glacial periods. However, using systematic variations in major element geochemistry, Sr–Nd isotopes and clay mineral records from Ocean Drilling Program Sites 1143 and 1144 in the South China Sea spanning the last 1.1 Ma, we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials. We attribute this to sub-aerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands. Our estimates suggest that enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands can account for 99% of the carbon dioxide removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial–inter-glacial variation of 980 ppmv. As a result, if similar magnitudes can be identified in other tropical shelf-slope systems, the effects of increased sediment exposure and subsequent silicate weathering during lowstands could have potentially enhanced

- by Peter Clift
- •
- Geochemistry, Climate Change, Marine Geology, Glaciations

Geochemical and palynological records are presented for an expanded Turonian–Coniacian hemipelagic succession in the central Bohemian Cretaceous Basin. A high-resolution stratigraphic framework is provided by biostratigraphy and organic carbon stable-isotope (δ 13 C org) chemostratigraphy. A short-term (100 kyr) sea-level curve has been derived from high-resolution transgressive/regressive maxima / shore-proximity data established from basin-wide sediment geometries. The viability of geochemical and palynological parameters as potential sea-level proxies is tested against this independently derived sea-level record. Elemental chemostratigraphy is demonstrated to offer a reliable means of identifying medium-to long-term (0.4–2.4 Myr) sea-level trends. Manganese maxima are associated with periods of high sea level, and troughs with intervals of low sea level. Falling Mn contents accompany regression and rising values transgression. Major transgressive events associated with medium-term sea-level change are marked by sharp increases in Ti/Al ratios, but short-term (100 kyr) sea-level cycles are not consistently identified. Long-term δ 13 C org variation and dinoflagellate cyst species richness are positively correlated and show similarities to the sea-level curve. Baseline trends have a cycle duration close to the 2.4 Myr long-eccentricity cycle. Dinocyst species richness closely follows short-term changes in sea level, with marked increases in dinocyst diversity coincident with most short-term flooding events. Periods of rapid sea-level rise caused an influx of a more diverse 'outer shelf' assemblage into the study area, together with the addition of shallower water species, some of which may have been transported into the central basin by hypopycnal flows. Changes in the proportion and abundance of peridinioid dinoflagellate cysts (principally Palaeohystrichophora infusorioides) were controlled principally by changing nutrient levels. Proximity proxies derived from geochemical and palynological data are not always consistent with the independent sea-level model. This exemplifies the need to understand all factors influencing elemental geochemical and palynological proxies before making simplistic sea level interpretations.

- by Darren Gröcke and +4
- •
- Geochemistry, Sedimentology, Stable Isotope Analysis, Palynology
- by Conor L Myhrvold
- •
- Geochemistry, Oceanography, Climate Change, Stable Isotope Analysis

La información geológica y geoquímica de la Cordillera Oriental Colombiana en este artículo de revisión inédita, corresponde a una síntesis de los importantes componentes geológicos tales como: las diversas facies sedimentarias cretácicas, eventos magmáticos como la presencia de gabros en la base del Cretácico y cuerpos volcánicos del paleógeno, además de los recursos minerales como los cinturones esmeraldíferos, los domos salinos y los prospectos, conocidos hasta el momento. Mientras que la Geoquímica se hace énfasis a lo desarrollado por Cartografía Geoquímica que se diferencia de la Prospección, al ser multi-objetivo y definir líneas base de composición química para materiales geológicos. Se hace un llamado a retomar estudios regionales con la quimio-estratigrafía como fundamento.

- by Sonia Güiza
- •
- Colombia, Trace element Geochemistry, Trace Metals, Sedimentary Geochemistry
- by Andreas Wetzel
- •
- Earth Sciences, Geology, Sedimentology, Geochronology

The Moss Ni-Mo-Zn-Pt-Pd-Re-Au hyper-enriched black shale (HEBS) showing is located in the western Richardson Mountains and is one of several in northern Yukon. The mineralization consists of a thin, stratiform semi-massive Fe-Ni-Mo-Zn sulphide horizon that occurs at the stratigraphic contact between the Road River Group and Canol Formation. This study evaluates the ambient paleoenvironmental conditions using several robust lithogeochemical proxies. Prior to HEBS formation, terrigenous clastic sedimentation predominated, whereas chemical sedimentation predominated during and immediately after HEBS formation. Rare earth element-Y data indicate that the water column was (weakly) oxygenated (Ce/Ce*SN < 1), that hydrothermal activity was absent (Eu/Eu*SN ≈ 1), and that there was a significant seawater influence on the sedimentary environment (Y/Ho > 28) throughout the deposition interval, even during HEBS mineralization. High (>10) authigenic Mo/U ratios suggest that a ferromanganese particulate shuttle delivered metals sourced from seawater to the seafloor. Negative bulk δ34S values (-19.3 to -23‰) in the HEBS indicate that microbially reduced seawater sulphate was the source of reduced sulphur for the mineralization. Collectively, these data signify a basinal environment that experienced varying degrees of restriction and stratification, but fresh (i.e. unfractionated) marine waters delivered metals, metalloids, and sulphur. This type of geological setting is considered critical for the formation and preservation of HEBS mineralization.

- by Carlos M González-León
- •
- Sedimentary Geochemistry

Karstification in Al-Kura District, northwestern of Jordan, is distribution in the Tertiary rocks at chalky and marley-limestone unit and it forms a local shallow unconfined aquifer. This research is constructed to describe and study the hydrogeochemical of karst features and their effects on the hydrochemistry of the Al-Dhaher Cave. Studied samples were collected from the water of wells and springs in the study area. All types of water have a composition in milligram units for cations (Ca2+>Mg2+>(Na++K+)), and anions (HCO3->Cl->NO3->SO42-). Calcium and bicarbonate accounts approximately 80% of the total ions. The concentrations of Ca2+ and Mg2+ are strongly correlated with HCO3-. The rCa2+/rMg2+ ratio for Al-Dhaher Spring is about 1.54, which suggests that water moves in chalky and marly limestone. Our data revealed that the main ions Ca2+, Mg2+, and HCO3- have very strong correlation to spring discharge. The water is undersaturated with respect to calcite and dolomite, and the correlation of SIc and SId to discharge is very strong (r=0.97 and 0.96). Depending on comparing the coefficients variations, the type of Al-Dhaher Spring could be classified as conduit spring. The results of water analyses studies show that the aquifer system is prone to karstification and they show the impact of karstification on the chemical composition of spring waters.

Geochemical data of sedimentary basins have been used to determine their plate tectonic setting. Here we test geochemical data for paired psammite–pelite samples from two greywacke packages that underlie 60% of the Paleozoic Acatlán Complex of southern Mexico: one an Ordovician rift-passive margin suite and the other a Carboniferous active margin suite. Both units consist of low-grade, polydeformed psammites and pelites associated with rare tholeiitic flows and dikes. Analyses of paired psammite–pelite samples from a
number of different units in the eastern part of the Acatlán Complex indicate that they all have a similar geochemistry with their major components dependent on the proportion of sand and clay. The chemical index of alteration yielded values between 55 and 70 recording a moderate degree of weathering of plagioclase to illite/kaolinite in the source area. The REE patterns suggest provenance in felsic–intermediate igneous rocks, and the wide range of TiO2–Zr ratios indicates that this was followed by extensive sorting. The mantle-normalized trace element patterns of the psammites generally display positive Zr and Hf anomalies reflecting elevated concentration of zircon in these rocks. Detrital zircon ages from some of these units show that the major source for these clastic rocks probably lay in the adjacent Mesoproterozoic Oaxacan Complex
and in the Neoproterozoic basement underlying the Acatlán Complex and/or the Yucatan Peninsula. Geological parameters point to a rift-passive margin setting for the Ordovician rocks and an active, subduction-related margin for the Carboniferous units. However, the commonly used tectonic discrimination,
bi- to tri-variant diagrams cannot readily distinguish between the two suites on geochemical grounds. Their geochemistry probably reflects the general felsic–intermediate compositions of the rocks in the Precambrian source regions, rather than the contemporaneous tectonic setting. Future work involving a
more advanced statistical approach (e.g. multivariant methods, multidimensional scaling and classification trees) may yield more refined results, particularly in combination with detrital zircon geochronology.

- by Jaroslav Dostal and +1
- •
- Geochemistry, Sedimentary provenance, Sedimentary Geochemistry
- by Eric E Hiatt and +3
- •
- Sedimentology, Stratigraphy, Sequence Stratigraphy, Sedimentary Geochemistry

Kurdistan Region is characterized among the rest of Iraqi territory, by the presence of metallic mineral deposits such as Zn, Pb, Cu, Cr-Ni and Mn, together with some unique non-metallic minerals and industrial rocks such as barite, asbestos and marble. The formation of these mineral deposits is associated with the geological and tectonic development of northern Iraq including the major tectonic events associated with the Tethys development and movement of the Arabian Plate. Most of the metallic mineralizations were developed during the Cretaceous Period. Some Zn-Pb deposits are older and may be related to the Triassic and Jurassic Periods. The actual mineral wealth of the region is not really known, in view of the lack of detailed geological mapping, geochemical survey and air-born geophysical survey. However, based on the available information it is expected that the province is promising for Zn, Pb, Cu, Cr, marble and barite deposits. Building raw materials are available to maint...

- by khaldoun al-bassam
- •
- Geology, Mineralogy, Sedimentary Geochemistry, Mineral Deposits geology

This paper discusses the advance of the speleothem luminescence
research. Potential, resolution and limitations of high resolution luminescence speleothem proxy records of
Paleotemperature, Solar Insolation, Solar Luminosity, Glaciations, Sea Level advances, Past Precipitation, Plants
Populations, Paleosoils, Past Karst Denudation, Chemical Pollution, Cosmic Rays Flux variations, Cosmogenic
Isotopes production and Supernova Eruptions in the Past, Advances of Hydrothermal Waters, and Techtonic Uplift are
discussed. It is demonstrated that speleothems allow extremely high resolution (higher than in any other paleoclimatic
terrestrial archives) and long duration of records. Some speleothems can be used as natural climatic stations for
obtaining of quantitative proxy records of Quaternary climates with annual resolution.

- by Yavor Shopov
- •
- Geography, Environmental Geography, Physical Geography, Palaeogeography

A lack of understanding exists of the origin and textural characteristics of Saudi Arabian Red Sea coastal sediments. This paper concerns the southern coastline of Jizan on the Saudi Red Sea. It is some 160 km long characterised by either narrow rocky headlands with intermittent pocket beaches or wide low-lying beaches dissected by wadis. Granulometric testing of samples from 135 locations showed that beach sand size was mainly very fine to medium grained (Mz = 3.93 Ø), sorting ranged from 1.65 to 0.41 and skewness values from −051 to 0.39, being mainly negative; dune sands were medium to fine grained (Mz = 1.13 Ø; average sorting 2.8), while skewness variations within dune samples indicated symmetrical to fine skewed values (бΙ = 0.55 to 0.89). Most foreshore samples were derived from wadis. Wadi mud levels can be high, e.g. Baysh (84%), and wadi Samrah (90%) with mean grain size ranging from very fine to medium sand (Mz = 3.9 Ø), sorting being well to poor (0.45 to 1.52) due to sediment influxes. Sabkha had a wide range of sand/mud and significantly higher carbonate percentages than other environments. Sediment source differences and littoral reworking contributed to grain size variation. The carbonate content varied between 1.5 and 31.5% due to hinterland contributions, and spatial analysis showed increasing quantities of carbonate minerals towards the south. On the wider geographical front, findings from Jizan are similar to those of the Northern United Arab Emirates (UAE), including sabkhas, being composed of sand, skeletal carbonate, fine fluvial material and wind-blown silt and clay components of wadi origin. Further work on the northeastern Red Sea edge can hopefully confirm these findings.

- by Omar Alharbi
- •
- Sedimentology, Coastal Processes, Coastal Geography, Coastal Geomorphology

Geochemical characteristics have been intensively used to assign sediment properties to paleoclimate and provenance. Nonetheless, in particular concerning the arid context, bulk geochemistry of different sediment archives and corresponding process interpretations are hitherto elusive. The Ejina Basin, with its suite of different sediment archives, is known as one of the main sources for the loess accumulation on the Chinese Loess Plateau. In order to understand mechanisms along this supra-regional sediment cascade , it is crucial to decipher the archive characteristics and formation processes. To address these issues, five profiles in different geomorphological contexts were selected. Analyses of X-ray fluorescence and diffraction, grain size, optically stimulated luminescence and radiocarbon dating were performed. Robust factor analysis was applied to reduce the attribute space to the process space of sedimentation history. Five sediment archives from three lithologic units exhibit geochemical characteristics as follows: (i) aeolian sands have high contents of Zr and Hf, whereas only Hf can be regarded as a valuable indicator to discriminate the coarse sand proportion; (ii) sandy loess has high Ca and Sr contents which both exhibit broad correlations with the medium to coarse silt proportions; (iii) lacustrine clays have high contents of felsic, ferromagnesian and mica source elements e.g., K, Fe, Ti, V, and Ni; (iv) fluvial sands have high contents of Mg, Cl and Na which may be enriched in evaporite minerals; (v) alluvial gravels have high contents of Cr which may originate from nearby Cr-rich bedrock. Temporal variations can be illustrated by four robust factors: weathering intensity, silicate-bearing mineral abundance, saline/alkaline magnitude and quasi-constant aeolian input. In summary, the bulk-composition of the late Quaternary sediments in this arid context is governed by the nature of the source terrain, weak chemical weathering, authigenic minerals, aeolian sand input, whereas pedogenesis and diagenesis exert only limited influences. Hence, this study demonstrates a practical geochemical strategy supplemented by grain size and mineralogical data, to discriminate sediment archives and thereafter enhance our ability to offer more intriguing information about the sedimentary processes in the arid central Asia.

- by Kaifeng Yu and +2
- •
- Sedimentary Geochemistry, Sedimentary geology and stratigraphy

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- by Mohamed Osman AWALEH
- •
- Geology, Stable Isotope Geochemistry, Sedimentary Geochemistry

The Himalayan and Tibetan highlands (mountains), with high rates of physical erosion, are extreme settings for earth surface processes, generating one of the largest recent terrigenous detritus and organic carbon discharges to the ocean. However, their significance with respect to the global carbon and climate cycles during the Quaternary is still unclear, especially in quantitative terms. Here, we present comprehensive records of continental erosion and weathering, terrestrial supply, marine productivity, and organic carbon burial in the distal Arabian Sea, Bay of Bengal, and southern South China Sea since ∼700 ka over orbital timescales. These records exhibit periodicities corresponding to sea level and Indian summer monsoon intensity changes. During glacial periods, the enhanced highland surface erosion and activation of deep-sea channels significantly increased inputs of terrigenous detritus, nutrients, and organic carbon into the Arabian Sea and Bay of Bengal, whereas strengthened continental shelf surface weathering and organic matter preservation occurred in the South China Sea. Conclusively, our integrative proxies in the study area demonstrate, for the first time, pronounced glacial burial pulses of organic carbon (∼1.12 × 10 12 mol/yr), dominantly originating from the highland surface erosion and marine productivity. Together with the increased silicate weathering on the exposed tropical continental shelves and in the tropical volcanic arcs, the enhanced burial flux of organic carbon in the tropical marginal seas, therefore, highlights the large contributions that tropical regions can make within the glacial-interglacial carbon inventory of the ocean and atmosphere and thus cause significant negative feedback on the global climate. Plain Language Summary Anthropogenic emissions of the greenhouse gas CO 2 are significantly changing the global climate and environment, resulting in a warmer state for which there is no historical analog. Marine records hold valuable lessons for the future of our warming world, as marine sediments are an important reservoir of the global organic carbon and then modulate release of CO 2 into the atmosphere. Currently, the major river systems originating from the Himalaya and Tibetan Plateau discharge ∼25% of the global fluvial sediment flux to the ocean, acting as an important source of continental organic carbon at tectonic and current timescales. Our integrative mineralogical-geochemical study demonstrates the enhanced highland (mountain) erosion and activation of deep-sea channels, increased supplies of the produced materials, strengthened marine productivity, and effective preservation of organic carbon in the deep Arabian Sea and Bay of Bengal during cold periods. In contrast, strengthened chemical decomposition of silicates on the exposed continental shelf was coeval with increased organic carbon storage in the deep South China Sea. The study area contributed ∼1/4 of the current global marine burial flux of organic carbon during sea-level lowstands and thus represents a key precedent for understanding increasingly severe global warming. XU ET AL.

Sedimentary records in the southwestern South China Sea reflect the evolving erosion and drainage systems that have operated in Southeast Asia during the Neogene. Analyses of the chemistry and clay mineral composition of sediments from International Ocean Discovery Program (IODP) Site U1433 allow us to examine these processes over the last 17 Ma. Sediment older than 8 Ma was deposited relatively slowly. Sr and Nd isotopes indicate a variable provenance with sequences of less and more altered material accompanied by strong changes in the proportion of smectite. Sediment flux was probably from Indochina, as well as from a more primitive volcanic source, most likely the Palawan ophiolite and/or Luzon. Sediments younger than 8 Ma show a more stable Sr and Nd isotope character, indicating sources close to those seen in the modern Mekong River, although with some influx from smaller rivers draining the Indochina margin especially from 4 to 8 Ma. Our data are consistent with seismic estimates for an onset to the Mekong in its present location after 8 Ma, following an avulsion from the Gulf of Thailand.

- by Peter Clift and +1
- •
- Marine Geology, Radiogenic Isotope Geochemistry, Sedimentary provenance, Sedimentary Geochemistry
- by Thomas Stevens
- •
- Geology, Geochemistry, Sedimentology, Stable Isotope Geochemistry

The production of biogenic silica has
dominated the marine silica cycle since early
Paleozoic time, drawing down the concentration
of dissolved silica in modern seawater
to a few parts per million (ppm). Prior to
the biological innovation of the fi rst silica
biomineralizing organisms in late Proterozoic
time, inputs of silica into Precambrian
seawater were balanced by strictly chemical
silica and silicate precipitation processes,
although the mechanics of this abiotic marine
silica cycle remain poorly understood.
Cherty sedimentary rocks are abundant in
Archean sequences, and many previous authors
have suggested that primary precipitation
of amorphous silica could have occurred
in Archean seawater. The recent discovery
that many pure chert layers in early Archean
rocks formed as sedimentary beds of sandsized,
subspherical silica granules has provided
direct evidence for primary silica
deposition. Here, we provide further sedimentological
and geochemical analyses of
early Archean silica granules in order to gain
a better understanding of the mechanisms of
granule formation. Silica granules are common
components of sedimentary cherts from
a variety of depositional settings and water
depths. The abundance and widespread distribution
of silica granules in Archean rocks
suggest that they represented a signifi cant
primary silica depositional mode and that
most formed by precipitation in the upper
part of the water column. The regular occurrence
of silica granules as centimeterscale
layers within banded chert alternating
with layers of black or ferruginous chert
containing few granules indicates episodic
granule sedimentation. Contrasting silicon
iso topic compositions of granules from different
depo sitional environments indicate
that isotopic signatures were modifi ed during
early diagenesis. Looking to modern siliceous
sinters for insight into silica precipitation, we
suggest that silica granules may have formed
via multiple stages of aggregation of silica
nanospheres and microspheres. Consistent
with this hypothesis, Archean ocean chemistry
would have favored particle aggregation
over gelling. Granule formation would have
been most favorable under conditions promoting
rapid silica polymerization, including
high salinity and/or high concentrations
of dissolved silica. Our observations suggest
that granule sedimentation was often episodic,
suggesting that granule formation may
have also been episodic, perhaps linked to
variations in these key parameters.

- by Elizabeth Trower
- •
- Sedimentology, Sedimentary Geochemistry, Precambrian Geology, Silica
- by Noelynna Ramos
- •
- Philippines, Geochronology, Sedimentary Petrography, Sedimentary Geochemistry

Reconstruction of paleo-redox conditions in a radiocarbon (14C) dated sediment core (SK-218/1), covering
the past 45 ka (thousand calendar years), collected from the western Bay of Bengal (Lat: 14° 02′N; Long:
82° 00′E) at a water depth of 3307 m, has been made based on redox-sensitive element geochemistry. The
high U/Th ratio, Mo enrichment, Mo/U enrichment factor ratio, Ce/Ce*b1 and lower Mn/Al and Fe/Al ratios,
compared to upper continental crust are all indicative of prevalence of suboxic condition in the benthic environment
from 15.2 ka to 4.5 ka, peaking around 9.5 ka. The suboxic condition around 9.5 ka corresponds
to the previously recorded southwest (SW) monsoon intensification in response to the increase in northern
hemisphere summer insulation. However, productivity proxies – organic carbon and nitrogen contents – do
not indicate marked increase in productivity at this time. It is proposed that as a result of large increase in
lithogenic material supplied from land due to SW monsoon intensification, which is evident by the very
high concentration of Al, Zr and Hf, the flux of fresh labile organic matter and these might have formed
dense mineral matter — biogenic aggregates which sinks rapidly to the seafloor, and the degradation of labile
organic matter might have led to the development of suboxic condition in the benthic environment. There
exists a strong positive correlation (r=0.98) between Mo and Zr during 15.2 ka to 4.5 ka suggesting a coupling
between suboxic condition and lithogenic flux supply by the intensified SW monsoon. Our results suggest
that temporal variability of the ballasting effect of the terrestrially-derived material could play a key role
in benthic biogeochemistry and ecology of the Bay of Bengal.
We also provide the first record of the nitrogen isotopic composition (δ15N) of sedimentary organic matter in
the western Bay of Bengal, a region where the mesopelagic oxygen minimum zone (OMZ) is just short of
being suboxic (denitrifying) today. The sedimentary δ15N fluctuated considerably in the past, especially during
the Marine Isotope Stage 3. Oscillations in δ15N were apparently in concert with those in organic carbon
and nitrogen contents and could be related to climatic changes (Heinrich and Dansgaard–Oeschger events) in
the North Atlantic. The Dansgaard–Oeschger 12 event appears to have exerted the most intense effect on Bay
of Bengal biogeochemistry when surface productivity, as inferred from the organic carbon and nitrogen contents,
was the highest recorded in the core, and the δ15N reached up to 6.3‰. Considering the probable dilution
by isotopically light terrigenous organic matter, it would appear that OMZ of the Bay of Bengal had
turned denitrifying. However, the absence of suboxic conditions in the sediments at this time suggests a
decoupling of the benthic processes with those in the mesopelagic water column.

- by Ishfaq Mir and +1
- •
- Sedimentary Geochemistry