Delta ¹³C depleted oceans before the termination 2: More nutrient-rich deep-water formation or light-carbon transfer? (original) (raw)
Related papers
Glacial to Holocene changes in the surface and deep waters of the northeast Indian Ocean
Marine Geology, 2012
Stable carbon and oxygen isotopic investigations are carried out on planktonic and benthic foraminifera from an AMS-dated sediment core of the northeast Indian Ocean (NEIO) to infer glacial to Holocene changes in surface and deep waters. The chronology of this gravity core (SK157-14; water-depth 3306 m; lat. 5°11'N; long. 90°05'E) was established using six AMS radiocarbon ages and oxygen isotope stratigraphy. Variations in δ18O and δ13C values of planktonic (Globigerinoides ruber) and benthic foraminifera (Cibicidoides spp.) are suggestive of large changes in the surface and deep water characteristics during the last ~ 60 ka. The δ18Opl values in core SK157-14 are significantly higher compared to the sediment cores in the northern Bay of Bengal and the Andaman Sea because of the diminished influence of riverine fresh water fluxes. Large variations in planktonic δ18Opl are noticed during the Marine Isotopic Stage (MIS) 3.1 and 3.3. Glacial to Holocene Δδ18Opl amplitude (1.8‰) is consistent with other published oxygen isotope records from the nearby locations. Maximum enrichment in δ18Opl occurs at 24-19 and the minimum during 7-6 ka BP. Spectral analysis of planktonic δ18Opl time series suggests a teleconnection between surface water δ18O and North Atlantic climate oscillations. Benthic foraminiferal δ18Oben values indicate deep water cooling of ~ 1.5 °C during the last glacial maximum. The δ13Cben values are generally higher for the Holocene foraminifera suggesting greater contribution from the North Atlantic Deep Water (NADW). However glacial benthic foraminifera are characterized by lower δ13Cben values. Highly depleted δ13Cben values during the ~ 60-50, 21-17 and 13-11 ka BP intervals suggest decrease contribution from the North Atlantic Deep Water (NADW) and increase influx from the Southern Ocean Deep Water (SODW). In addition, oxidation of organic matter and ageing of the deep water may have contributed in the pronounced decrease in δ13Cben during the glacial intervals.
Indian Ocean circulation and productivity during the last glacial cycle
Earth and Planetary Science Letters, 2009
The Indian Ocean is an important part of the global thermohaline circulation system, receiving deep waters sourced from the Southern Ocean and being the location of upwelling and surface-ocean current flow, which returns warm and salty waters to the Atlantic. It is also an ideal location to reconstruct the link between thermohaline circulation and deep-water nutrient contents. No mixing occurs between major deep-water masses along flow paths within the Indian Ocean, so changes in water-mass provenance reflect changes in deep-ocean circulation while nutrient contents reflect addition and dissolution of organic matter. We present neodymium (Nd) and carbon (C) isotope records, proxies of water-mass provenance and nutrient contents, respectively, from an equatorial Indian Ocean core (SK129−CR2) spanning the last 150 kyr. The Nd isotope record shows that an increased proportion of North Atlantic Deep Water (NADW) reached the Indian Ocean during interglacials (marine isotope stages, MIS 1 and 5), and a reduced proportion during glacials (MIS 2, 4, and 6), and also that changes occurred during MIS 3. The magnitude and timing of deglacial and some MIS 3 variability is very similar to those in the RC11−83/TNO57−21 South Atlantic deep Cape Basin Nd isotope record, suggesting that Atlantic meridional overturning circulation changes were effectively propagated from the southeastern Atlantic into the central Indian Ocean via the Southern Ocean. Comparison of the Nd and C isotope records shows that deep-ocean circulation was decoupled from nutrient-content changes on glacial−interglacial timescales, in particular suggesting higher productivity during MIS 5.
Palaeogeography, Palaeoclimatology, Palaeoecology, 1999
Palaeoceanographic conditions in the eastern Indian Ocean for the last ∼30 kyr are documented by means of planktonic foraminiferal analyses of 10 gravity cores. Quantitative foraminiferal analysis (%), Q-mode factor analysis, the modern analog technique (MAT) and oxygen-isotope analyses are used. A conspicuous increase during the last glacial maximum (LGM) of foraminiferal fragmentation resulting from a more productive Java upwelling system and/or a more corrosive Antarctic Intermediate Water (AAIW) was found at intermediate water depths (∼1000 m). Contrasting Q-mode factors based on foraminifera between today and the LGM suggest changes in the thermocline depth, sea-surface temperature (SST), upwelling, and the strength of both the Australasian Mediterranean Water (AAMW) and the Indian Central Water (ICW). The decrease in the percentage abundance of shallow-dwelling and symbiont-bearing planktonic foraminifera, the increase in percentage of the upwelling-related species Globorotalia cultrata and Neogloboquadrina dutertrei, and factor 3 (dominated by Globorotalia tumida and Globigerinella siphonifera) suggest a stronger Java upwelling system during the LGM. A steeper, steric latitudinal gradient (in the presence of a weak Leeuwin Current), and a geostrophic flow similar to today's is postulated for the LGM, and this must have prevented upwelling offshore Western Australia. Today's AAMW–ICW sharp front was weaker during the LGM when the AAMW was saltier, cooler, and nutrient richer and more similar to the ICW. During the LGM, a more gentle SST latitudinal gradient over the ∼16 to ∼23°S region contrasts with today's steeper conditions at the AAMW–ICW Front. Also, for the LGM, a nutrient-rich ICW may explain previously documented increases in mass accumulation rates of CaCO3, organic carbon and benthonic foraminifera in a region where the nutricline was deep and within the lower euphotic zone.
Journal of Asian Earth Sciences, 2019
Here we present the first detailed planktonic and benthic 13 C records and benthic foraminiferal assemblage records from the northeastern Indian Ocean to decipher the palaeoceanographic changes during the last 56 kyr. We identified three different palaeoceanographic stages, clearly differentiated by significant variations in the benthic foraminiferal assemblages and 13 C records. The results of this study indicate that productivity was generally higher during the glacial periods than during the Holocene. Comparison of the benthic foraminiferal assemblage distributions and planktonic and benthic 13 C records show a significant correlation between productivity and the bottom water oxygenation on glacial-interglacial timescales. Productivity gradually increased during the period between 56-27.5 kyr. During this period, the dominance of Melonis spp. and Oridosalis umbonatus were correlated with conditions of intermediate to high surface productivity and moderate bottom water oxygenation. Increased higher equatorial productivity and low bottom water oxygenation during the period between 27.5-15 kyr are supported by planktonic 13 C and faunal records. During this period, the dominance of benthic foraminifera assemblages characterised by Uvigerina peregrina indicates sustained continuous phytodetritus flux to the seafloor from enhanced surface water productivity and relatively low bottom water oxygenation. The absence or minimal occurrences of high productivity indicating U. peregrina, the dominance of intermediate to low productivity indicating fauna, and relatively low planktonic 13 C records suggest low productivity and active deep-water oxygenation after 15 kyr. Concurrent river discharge and rising sea levels during this period are indicated by negative 18 O of G. ruber at the site.
Geochemistry, Geophysics, Geosystems, 2019
Benthic foraminiferal assemblages and geochemical tracers (δ 18 O, δ 13 C and 14 C) have been analyzed on benthic and planktonic foraminifera from core MD77-176, located in the northern Bay of Bengal, in order to reconstruct the evolution of intermediate circulation in the northern Indian Ocean since the last glaciation. Results indicate that during the Last Glacial Maximum (LGM), Southern Sourced Water masses were dominant at the core site. A high relative abundance of intermediate and deep infaunal species during the LGM reflects low oxygen concentration and/or mesotropic to eutrophic deep water conditions, associated with depleted benthic δ 13 C values. During the Holocene, benthic foraminiferal assemblages indicate an oligotropic to mesotrophic environment with well-ventilated bottom water conditions compared with LGM. Higher values for benthic foraminifera δ 13 C and B-P 14 C age offsets suggest an increased contribution of North Atlantic Deep Water to the northern Bay of Bengal during the Late Holocene compared to the LGM. Millennial-scale events punctuated the last deglaciation, with a shift in the δ 13 C and the ɛ Nd values coincident with low B-P 14 C age offsets, providing strong evidence for an increased contribution of Antarctic Intermediate Water at the studied site. This was associated with enhanced upwelling in the Southern Ocean, reflecting a strong sea-atmospheric CO 2 exchange through Southern Ocean ventilation during the last deglaciation.
Journal of Asian Earth Sciences, 2006
This study attempts to analyse paleoceanographic changes in the Central Indian Ocean (Deep Sea Drilling Project Site 237), linked to monsoon variability as well as deep-sea circulation during the Plio-Pleistocene. We used factor and cluster analyses of census data of the 34 most dominant species of benthic foraminifera that enabled us to identify five biofacies: Astrononion umbilicatulum-Uvigerina proboscidea (Au-Up), Pullenia bulloides-Bulimina striata (Pb-Bs), Globocassidulina tumida-Nuttallides umbonifera (Gt-Nu), Gyroidinoides nitidula-Cibicides wuellerstorfi (Gn-Cw) and Cassidulina carinata-Cassidulina laevigata (Cc-Cl) biofacies. Knowledge of the environmental preferences of modern deep-sea benthic foraminifera helped to interpret the results of factor and cluster analyses in combination with oxygen and carbon isotope values. The biofacies indicative of high surface productivity, resulting from a stronger South Equatorial Current (Au-Up and Pb-Bs biofacies), dominate the early Pliocene interval (5.6-4.5 Ma) of global warmth. An intense Indo-Pacific 'biogenic bloom' and strong Oxygen Minimum Zone extended to intermediate depths (w1000-2000 m) over large parts of the Indian Ocean in the early Pliocene. Since 4.5 Ma, the food supply in the Central Indian Ocean dropped and fluctuated while deep waters were corrosive (biofacies Gt-Nu, Gn-Cw). The Pleistocene interval is characterized by an intermediate flux of organic matter (Cc-Cl biofacies). q
Climate of The Past, 2022
We have measured Cd/Ca ratios of several benthic foraminiferal species and studied benthic foraminiferal assemblages on two cores from the northern Indian Ocean (Arabian Sea and northern Bay of Bengal, BoB), in order to reconstruct variations in intermediate-water circulation and paleo-nutrient content since the last deglaciation. Intermediate water Cd w records estimated from the benthic Cd/Ca reflect past changes in surface productivity and/or intermediate-bottom-water ventilation. The benthic foraminiferal assemblages are consistent with the geochemical data. These results suggest that during the last deglaciation, Cd w variability was primarily driven by changes in intermediate-water properties, indicating an enhanced ventilation of intermediate-bottom water masses during both Heinrich Stadial 1 and the Younger Dryas (HS1 and YD, respectively). During the Holocene, however, surface primary productivity appears to have influenced Cd w more than intermediate water mass properties. This is evident during the early Holocene (from 10 to 6 cal ka) when benthic foraminiferal assemblages indicate that surface primary productivity was low, resulting in low intermediate-water Cd w at both sites. Then, from ∼ 5.2 to 2.4 cal ka, surface productivity increased markedly, causing a significant increase in the intermediate-water Cd w in the southeastern Arabian Sea and the northeastern BoB. The comparison of intermediatewater Cd w records with previous reconstructions of past Indian monsoon evolution during the Holocene suggests a direct control of intermediate-water Cd w by monsoon-induced changes in upper-water stratification and surface primary productivity.
2005
Carbon-isotopes (δ 13 C) composition of benthic foraminifera has been extensively used to understand the link between deep-water circulation and climate. Equatorial Indian Ocean δ 13 C records of planktic-and benthic-foraminifera together show an unexplained shift in the long-term mean oceanic-δ 13 C around the penultimate glacial termination (T2: 132 ka). The time-series planktic-and benthic-species δ 13 C records exhibit two distinct mean-δ 13 C levels. The low mean-δ 13 C characterises the pre-T2 period (250 ka -132 ka), while the post-T2 (~95 ka -Present) period records high mean-δ 13 C, generating a one-time shift of ~0.4 ‰ within the last ~250 kyr time-period. This shift is a result of consistently higher-δ 13 C in post-T2 glacial (and interglacial) periods as compared to the pre-T2 glacial (and interglacial) periods, and begins around the T2 (~132 ka), lasts until ~95 ka, and sustained through the T1. The normally observed glacial-interglacial δ 13 C variations of ~0.3 ‰ occur as secondary fluctuations around the long-term primary mean-levels in the Indian Ocean, as well as in other oceans. The T2-δ 13 C shift appears to be an inherent feature of the world oceans although with certain timing offsets. Therefore, it should represent a fundamental change in deep-ocean circulation (nutrient) dynamics. But, the leading hypotheses of circulation-driven oceanic distribution of δ 13 C fail to explain the observed mean-δ 13 C shift. Therefore it is proposed that, in addition to changes in deep-water circulation, the oceans before T2 were characterised by significantly lower-δ 13 C than after. Such low-δ 13 C mean-ocean during the pre-T2 period might have been the result of significantly increased transfer of terrestrial light-carbon to the ocean reservoir due to changes in global wind patterns.
2005
Carbon-isotopes (δ 13 C) composition of benthic foraminifera has been extensively used to understand the link between deep-water circulation and climate. Equatorial Indian Ocean δ 13 C records of planktic-and benthic-foraminifera together show an unexplained shift in the long-term mean oceanic-δ 13 C around the penultimate glacial termination (T2: 132 ka). The time-series planktic-and benthic-species δ 13 C records exhibit two distinct mean-δ 13 C levels. The low mean-δ 13 C characterises the pre-T2 period (250 ka-132 ka), while the post-T2 (~95 ka-Present) period records high mean-δ 13 C, generating a one-time shift of ~0.4 ‰ within the last ~250 kyr time-period. This shift is a result of consistently higher-δ 13 C in post-T2 glacial (and interglacial) periods as compared to the pre-T2 glacial (and interglacial) periods, and begins around the T2 (~132 ka), lasts until ~95 ka, and sustained through the T1. The normally observed glacial-interglacial δ 13 C variations of ~0.3 ‰ occur as secondary fluctuations around the long-term primary mean-levels in the Indian Ocean, as well as in other oceans. The T2-δ 13 C shift appears to be an inherent feature of the world oceans although with certain timing offsets. Therefore, it should represent a fundamental change in deep-ocean circulation (nutrient) dynamics. But, the leading hypotheses of circulation-driven oceanic distribution of δ 13 C fail to explain the observed mean-δ 13 C shift. Therefore it is proposed that, in addition to changes in deep-water circulation, the oceans before T2 were characterised by significantly lower-δ 13 C than after. Such low-δ 13 C mean-ocean during the pre-T2 period might have been the result of significantly increased transfer of terrestrial light-carbon to the ocean reservoir due to changes in global wind patterns.
Palaeogeography Palaeoclimatology Palaeoecology, 2008
Tropical climate is variable on astronomical time scale, driving changes in surface and deep-sea fauna during the Pliocene-Pleistocene. To understand these changes in the tropical Indian Ocean over the past 2.36 Myr, we quantitatively analyzed deep-sea benthic foraminifera and selected planktic foraminifera from N 125 μm size fraction from Deep Sea Drilling Project Site 219. The data from Site 219 was combined with published foraminiferal and isotope data from Site 214, eastern Indian Ocean to determine the nature of changes. Factor and cluster analyses of the 28 highest-ranked species distinguished four biofacies, characterizing distinct deep-sea environmental settings. These biofacies have been named after their most dominant species such as Stilostomella lepidula-Pleurostomella alternans (Sl-Pa), Nuttallides umbonifer-Globocassidulina subglobosa (Nu-Gs), Oridorsalis umbonatus-Gavelinopsis lobatulus (Ou-Gl) and Epistominella exigua-Uvigerina hispido-costata (Ee-Uh) biofacies. Biofacies Sl-Pa ranges from~2.36 to 0.55 Myr, biofacies Nu-Gs ranges from~1.9 to 0.65 Myr, biofacies Ou-Gl ranges from~1 to 0.35 Myr and biofacies Ee-Uh ranges from 1.1 to 0.25 Myr. The proxy record indicates fluctuating tropical environmental conditions such as oxygenation, surface productivity and organic food supply. These changes appear to have been driven by changes in monsoonal wind intensity related to glacial-interglacial cycles. A shift at~1.2-0.9 Myr is observed in both the faunal and isotope records at Site 219, indicating a major increase in monsoon-induced productivity. This coincides with increased amplitude of glacial cycles, which appear to have influenced low latitude monsoonal climate as well as deep-sea conditions in the tropical Indian Ocean.