Gerald Ganssen - Academia.edu (original) (raw)

Papers by Gerald Ganssen

Holes 603C and 604 of DSDP Leg 93 were drilled on the western Atlantic continental rise at water ... more Holes 603C and 604 of DSDP Leg 93 were drilled on the western Atlantic continental rise at water depths of 4633 m and 2364 m, respectively. In Hole 603C, a nearly continuous, undisturbed, and complete section of Pliocene and lower Pleistocene sediments was recovered by hydraulic piston coring; in Hole 604, a section of uppermost Miocene to Pleistocene sediments was incompletely recovered by rotary coring.In order to reconstruct the Pliocene and Pleistocene history of isotopic variations, 139 oxygen and carbon isotope values were determined for planktonic and monospecific benthic foraminifer samples from these holes. Large parts of the Pleistocene history could not, however, be documented because sample intervals were large and sediments at Site 604 were redeposited. Time correlation is based on magnetostratigraphic (Hole 603C) and micropaleontologic (Hole 603C, Site 604) interpretation.Stable isotope analyses were carried out on the planktonic foraminiferal species Globigerinoides r...

Nooteboom et al., use a strongly eddying global ocean model simulation to investigate the influen... more Nooteboom et al., use a strongly eddying global ocean model simulation to investigate the influence of particle advection by ocean currents on sedimentary microplankton distributions.

PAGES news, 2010

Periodic intensifications of Antarctic Intermediate Water flow occurred as part of the millennial... more Periodic intensifications of Antarctic Intermediate Water flow occurred as part of the millennial-scale climate oscillations in the glacial period.

Climate of the Past Discussions, 2019

Changeover from a glacial to an interglacial climate is considered as transitional between two st... more Changeover from a glacial to an interglacial climate is considered as transitional between two stable modes. Palaeoceanographic reconstructions using the polar foraminifera Neogloboquadrina pachyderma highlight the retreat of 15 the polar front during the last deglaciation in terms of both its decreasing abundance and stable oxygen isotope values (δ 18 O) in sediment cores. While conventional isotope analysis of pooled N. pachyderma shells show a warming trend concurrent with the retreating ice, new single shell measurements reveal that this trend is composed of two isotopically different populations that are morphologically indistinguishable. Using modern time-series as analogues for interpreting down-core data, glacial productivity in the mid North Atlantic appears limited to a single maximum in late summer, 20 followed by the melting of drifting icebergs and winter sea ice. Despite collapsing ice sheets and global warming during the deglaciation a second 'warm' population of N. pachyderma appears in a bimodal seasonal succession separated by the subpolar G. bulloides. This represents a shift in the timing of the main plankton bloom from late to early summer in a deglacial intermediate mode that persisted for ca. 10,000 years until the last deglaciation ended. When seawater temperatures exceeded the threshold values, first the "cold" (glacial) then the "warm" (deglacial) population of N. 25 pachyderma disappeared, whilst G. bulloides with a greater tolerance to higher temperatures persisted throughout the Holocene to the present day in the mid-latitude North Atlantic. Single specimen δ 18 O of polar N. pachyderma reveal a steeper rate of ocean warming during the last deglaciation than appears from conventional pooled δ 18 O average values.

Biogeosciences Discussions, 2019

The West Antarctic Peninsula (WAP) exhibits strong spatial and temporal oceanographic variability... more The West Antarctic Peninsula (WAP) exhibits strong spatial and temporal oceanographic variability, resulting in highly heterogeneous biological productivity. Calcifying organisms that live in the waters off the WAP respond to temporal and spatial variations in ocean temperature and chemistry. These marine calcifiers are potentially threatened by regional climate change with waters already naturally close to carbonate undersaturation. Future projections of carbonate production in the Southern Ocean are challenging due to the lack of historical data collection and complex, decadal climate variability. Here we present a six-year long record of the shell fluxes, morphology, and stable isotope variability of the polar planktic foraminifera Neogloboquadrina pachyderma (sensu stricto) from near Palmer Station, Antarctica. This species is fundamental to Southern Ocean planktic carbonate production as it is one of the very few planktic foraminifer species adapted to the marine polar environments. We use these new data to obtain insights into its ecology and to derive a robust assessment of the response of this polar species to environmental change. Morphology and stable isotope composition reveal the presence of different growth stages within this tightly defined species. Inter-and intra-annual variability of foraminiferal flux and size is evident and driven by a combination of environmental forcing parameters, most importantly food availability, temperature, and sea-ice duration and extent. Foraminiferal growth occurs throughout the austral year and is influenced by environmental change, a large portion of which is driven by the Southern Annular Mode and El Niño Southern Oscillation. A distinct seasonal production is observed, with highest shell fluxes during the warmest and most productive months of the year. The sensitivity of calcifying foraminifera to environmental variability in this region, from weeks to Biogeosciences Discuss.,

Marine Ecology Progress Series, 2018

Young adult atlantid heteropod Oxygyrus inflatus collected in the Atlantic Ocean during cruise AM... more Young adult atlantid heteropod Oxygyrus inflatus collected in the Atlantic Ocean during cruise AMT27. Maximum shell diameter (excluding keel) 1.6 mm.

Radiocarbon, 1995

We quantified the rate of carbonate dissolution with increasing water depth by taking the differe... more We quantified the rate of carbonate dissolution with increasing water depth by taking the difference in the carbonate mass accumulation rate of deep (3393–4375 m) core top sediments from the shallowest one (3208 m), which we assumed was unaffected by dissolution. This method depends on high quality14C dates that we calibrated to calendar years for calculating sedimentation rates. Our results show low (ranging from 0 to 0.3 g cm−2ka−1) and high (ranging from 1.5 to 1.7 g cm−2ka−1) carbonate dissolution rates, above and below 4000 m, respectively. Therefore, we interpret the sudden increase in the carbonate dissolution rate at 4000-m water depth to mark the lysocline.

Paleoceanography, 2016

The Open University's repository of research publications and other research outputs Geochemical ... more The Open University's repository of research publications and other research outputs Geochemical imprints of genotypic variants of Globigerina bulloides in the Arabian Sea

Biogeosciences, 2015

Most planktonic foraminifera migrate vertically through the water column during life, meeting a r... more Most planktonic foraminifera migrate vertically through the water column during life, meeting a range of depth-related conditions as they grow and calcify. For reconstructing past ocean conditions from geochemical signals recorded in their shells, it is therefore necessary to know vertical habitat preferences. Species with a shallow habitat and limited vertical migration will reflect conditions of the surface mixed layer and short-term and mesoscale (i.e. seasonal) perturbations therein. Species spanning a wider range of depth habitats, however, will contain a more heterogeneous, intra-specimen variability (e.g. Mg / Ca and δ 18 O), which is less for species calcifying below the thermocline. Obtained single-chamber Mg / Ca ratios are combined with singlespecimen δ 18 O and δ 13 C of the surface-water inhabitant Globigerinoides ruber, the thermocline-dwelling Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, and the deep dweller Globorotalia scitula from the Mozambique Channel. Species-specific Mg / Ca, δ 13 C and δ 18 O data combined with a depth-resolved mass balance model confirm distinctive migration and calcification patterns for each species as a function of hydrography. Whereas single-specimen δ 18 O rarely reflects changes in depth habitat related to hydrography (e.g. temperature), measured Mg / Ca of the last chambers can only be explained by active migration in response to changes in temperature stratification. Foraminiferal geo-chemistry and modelled depth habitats shows that the singlechamber Mg / Ca and single shell δ 18 O are in agreement with each other and in line with the changes in hydrography induced by eddies.

Paleoceanography, 2015

Geochemical and morphological characteristics of Globorotalia truncatulinoides, a deep dwelling p... more Geochemical and morphological characteristics of Globorotalia truncatulinoides, a deep dwelling planktonic foraminifer, have been used since the mid‐1950s to infer (paleo)oceanographic conditions of the upper ocean. The coiling ratio has been linked to different water masses and stable oxygen isotope signal of this species to changes in depth habitat and/or season. Here we show that the isotopic composition of single specimens covering Termination III of multiple size fractions of North Atlantic G. truncatulinoidessinistral is indicative of a deeper calcification depth in the water column compared to G. truncatulinoidesdextral as previously indirectly inferred in a plankton tow study. Furthermore, the change in coiling ratio from dominantly G. truncatulinoidesdextral (95%) to brief episodes of dominantly G. truncatulinoidessinistral (80%) gives a strong indication of deepening of the permanent thermocline during periods in which G. truncatulinoidessinistral was dominant. The positio...

Climate of the Past Discussions, 2014

Millennial scale climate variability punctuates the record of Northern Hemisphere glacials in the... more Millennial scale climate variability punctuates the record of Northern Hemisphere glacials in the form of Dansgaard–Oeschger cycles. The coldest episodes within these cycles have a marked impact on the location of the Inter-tropical Convergence Zone and thereby the intensity and extent of the Asian monsoon system. Arabian Sea oceanography is closely linked to the monsoon cycle and responds rapidly to changes therein. Seasonally varying upwelling intensity, adhesive mixing and the resulting variability on sea surface temperature and salinity, however, make it challenging to precisely reconstruct the behaviour of the Indian monsoon system through the Dansgaard–Oeschger cycles. To reconstruct impact of millennial-scale changes in the monsoon on the Arabian Sea, paired single-specimen Mg/Ca and stable oxygen isotope analyses were performed on three species of planktonic foraminifera from the northern as well as the western Arabian Sea. Mismatches between Mg/Ca- and δ<sup>18</su...

Paleoceanography, 1994

Using 95 epibenthic δ13C records, eight time slices were reconstructed to trace the distribution ... more Using 95 epibenthic δ13C records, eight time slices were reconstructed to trace the distribution of east Atlantic deepwater and intermediate water masses over the last 30,000 years. Our results show that there have been three distinct modes of deepwater circulation: Near the stage 3‐2 boundary, the origin of North Atlantic Deep Water (NADW) was similar to today (mode 1). However, after late stage 3 the source region of the NADW end‐member shifted from the Norwegian‐Greenland Sea to areas south of Iceland (mode 2). A reduced NADW flow persisted during the last glacial maximum, with constant preformed δ13C values. The nutrient content of NADW increased markedly near the Azores fracture zone from north to south, probably because of the mixing of upwelled Antarctic Bottom Water (AABW) from below, which then advected with much higher flux rates into the northeast Atlantic. Later, the spread of glacial meltwater over the North Atlantic led to a marked short‐term ventilation minimum below ...

Paleoceanography, 2001

We present Holocene stable oxygen isotope data from the deep Arabian Sea off Somalia at a decadal... more We present Holocene stable oxygen isotope data from the deep Arabian Sea off Somalia at a decadal time resolution as a proxy for the history of intermediate/upper deep water. These data show an overall b180 reduction by 0.5%0 between 10 and •6.5 kyr B.P. superimposed upon short-term b180 variations at a decadal-centennial timescale. The amplitude of the decadal variations is 0.3%0 prior, and up to 0.6%0 subsequent, to •8.1 kyr B.P. We conclude from modeling experiments that the short-term •80 variations between 10 and •6.5 kyr B.P. most likely document changes in the evaporation-precipitation balance in the central Red Sea. Changes in water temperature and salinity cause the outflowing Red Sea Water to settle roughly 800 m deeper than today. 1. Introduction In order to improve our understanding of the processes driving climate change, a detailed knowledge of coupled oceanic-atmospheric dynamics is crucial. There are a number of strategic locations where variations in the evaporation-precipitation balance of marginal basins should affect the formation of intermediate water masses. The hypersaline Red Sea is a good example. To date, studies of the history of the water exchange between the Red and Arabian Seas have been carded out only from a Red Sea perspective. The intermediate/upper deepwater history in the Arabian Sea has received little attention. Variations in the b i80 gradient in the Arabian Sea water column (e.g., between planktic and benthic foraminifera) have been attributed to changes in the thermocline thickness due to monsoonal strength variations [Niitsuma et al., 1991 ], rather than to variations in Red Sea Water (RSW) outflow. On the basis of b•80 variations in benthic foraminifera from the Oman Margin, Zahn and Pedersen [1991] were the first to conclude that owing to the lower glacial sea level, the sill depth at the Strait of Bab el Mandeb shoaled (Figure 1). Accordingly, the water exchange between the Red and Arabian Seas was reduced, and offshore Oman waters forming in the Arabian Sea replaced RSW. The restricted water exchange also resulted in salinities higher than 50 practical salinity units (psu) during glacial stages [Hemleben et al., 1996; Rohling, 1994; Thunell et al., 1988]. Modem water exchange patterns between the Arabian and Red Seas were not established prior to 6-7.5 ka [Locke and Thunell, 1988]. This implies that the early Holocene salinity values in the Red Sea were much higher than those today and must have influenced the history of early Holocene outflow. Here we present b•aO data from middepth Arabian Sea, which document a shortterm variability that can be explained by a variable inflow of RSW with unusual temperature-salinity (T-S) properties during the early Holocene. 2. Hydrographic Setting of the Arabian Sea Below the monsoon-controlled surface mixed layer, four major intermediate to deep water masses dominate the Arabian Sea, i.e., Arabian Sea Water (ASW), Persian Gulf Water (PGW), Red Sea

Paleoceanography, 1991

Hydrographic data and oxygen isotopic analyses performed on surface waters and planktonic foramin... more Hydrographic data and oxygen isotopic analyses performed on surface waters and planktonic foraminiferal tests, collected during early summer from two succeeding years (1984, 1985) throughout the Red Sea, reveal two different hydrographic regimes. In 1984 the summer “normal” situation prevailed where surface waters from the Red Sea flowed out into the Gulf of Aden, while in 1985 a reversed inflow current occurred. The higher temperatures and salinities observed in 1985 indicate high evaporation rates and increased aridity in the northern Red Sea and caused this inflow of Indian Ocean Surface Water which origins from the active upwelling region in the Arabian Sea. Lower salinities and lower oxygen isotopes were observed up to 18°N. The occurrence of Globorotalia menardii during 1985 with its surprisingly constant isotope values up to the Gulf of Suez indicates northward flowing surface currents for the entire Red Sea. Isotope values from Neogloboquadrina dutertrei (1985) indicate subs...

Nature, 1998

well as through the large-field-gradient positions just above and below the crown-ether molecules... more well as through the large-field-gradient positions just above and below the crown-ether molecules. We have also made direct measurements of the ionic conductivity of the crown-ether salt and also of reference materials. We used an electron-blocking method, in which a compacted disk of the material under test was sandwiched between Li + /polyethylene oxide disks which were pressed onto Li metal electrodes. The ionic conductivity of (PEG)-Li(CF 3 SO 3), where PEG is polyethylene glycol, was measured to be 1:6 ϫ 10 Ϫ 6 S cm Ϫ 1 at 60 ЊC, in good agreement with literature values 15. We measured values, also at 60 ЊC, for a range of Ni(dmit) 2 salts: 3 ϫ 10 Ϫ 6 S cm Ϫ 1 for Li 0.6 (15-crown-5-ether) [Ni(dmit) 2 ] 2 ⋅H 2 O; 2 ϫ 10 Ϫ 8 S cm Ϫ 1 for (C 16 H 33) 2 (CH 3) 2 N[Ni(dmit) 2 ]; and 3 ϫ 10 Ϫ 7 S cm Ϫ 1 for (TPP)[Ni(dmit) 2 ] 3 , where TPP is tetraphenylphosphonium. Each measurement was made after the current reached its saturated value, typically between 30 and 60 minutes after application of the constant voltage. We have checked that there is no decomposition (for example, water loss) at 60 ЊC (by thermal gravimetry), and consider that the value of ionic conductivity for the crown-ether Li + salt indicates clearly that there is significant Li + mobility in this material at this temperature. We have measured significantly lower values in the (electronically insulating) tetra-alkyl ammonium salt and the (electronically conducting) tetraphenylphosphonium salt. We consider that the electron dynamics on the Ni(dmit) 2 chains are controlled by the dynamics of the Li + ions in the crown-ether columns. Above 250 K the Li + ions are mobile, and do not localize the conduction electrons on the Ni(dmit) 2 stacks, which therefore show metallic properties. Between 250 and 200 K the Li + ions become immobilized, and provide static coulombic pinning potentials for the conduction electrons on the Ni(dmit) 2 stacks, causing localization and the magnetic insulator properties evident in Figs 2 and 3. A metal-insulator transition driven by interaction between ions and electrons in this way is unusual, and the example that we have illustrated here may provide insights into transport properties of other mixed ionic/electronic conductors, such as the conjugated polymer blends recently used by Pei et al. 5 in electroluminescent diodes. The electron-transport system with ion channels that we describe here may allow for the design of electron-cation transport systems for use in active and selective ion transport. For example, a double carrier process could be realised in a redox gradient where the coupled parallel transport of electrons and alkali-metal cations takes place 16 .

Marine Micropaleontology, 1999

Planktic foraminiferal faunas from different environments in the Arabian Sea were size fractionat... more Planktic foraminiferal faunas from different environments in the Arabian Sea were size fractionated using 14 sieves with meshes between 100 and 710 µm, to assess the effect of the sieve mesh size cut off level on the faunal composition and to determine the size frequency distribution of individual species. Nine samples from a plankton pump and a towed net, a sediment trap, a box-core and a piston core were selected, to cover living and settling flux faunas as well as fossil faunas from the sediment. In living faunas, most species show an exponential size frequency distribution, with highest numbers in the finest interval of the size spectrum. In sediment trap and core samples, individual species size frequency distributions may consist of: (1) an exponential distribution of relatively small pre-adult specimens; (2) a Gaussian-shaped distribution of larger specimens, which may be classified as adult or terminal; or (3) a combination of both. The distributions are separated using a best fit technique. The composition of the total planktic foraminiferal fauna strongly changes along the size spectrum. Dominant taxa in >355 µm fractions are Orbulina universa, Globorotalia menardii, Globorotalia tumida, Globigerinella siphonifera and Globigerinoides sacculifer, in 125-355 µm fractions Globigerina bulloides, Globigerinoides ruber, Neogloboquadrina dutertrei and Globigerinita glutinata, and in <125 µm fractions Dentigloborotalia anfracta, Tenuitella compressa, Tenuitella iota, Turborotalita quinqueloba and the immature specimens of larger species. Consequently, the choice of the sieve mesh size strongly determines the percent composition of the assemblage and in turn the paleoceanographic interpretations based on these counts. Species richness and the Shannon diversity increase with decreasing sieve mesh size, while equitability generally decreases with decreasing size. In the water column approximately 60% of the fauna (>100 µm) is present in the 100-125 µm fraction and 1-6% is larger than 250 µm. In samples representing a settling flux (sediment trap and sediment samples) 29-57% of the fauna is present in the 100-125 µm fraction, while 6-23% is larger than 250 µm. Size frequency distributions of the dextral Neogloboquadrina complex (D Neogloboquadrina dutertrei and Neogloboquadrina pachyderma C P-D intergrades) show a bimodal pattern; a smaller peak reflecting dextral Neogloboquadrina pachyderma, and a larger peak of adult Neogloboquadrina dutertrei. By applying a best fit technique to the data, the two species may be separated from each other. In size fractions larger than 150 µm most species have reached the adult stage of ontogeny and we recommend this mesh size for standard faunal analysis. In addition, sieve mesh sizes of 125 and 250 µm have to be used to obtain a reliable estimate of the abundance of small and large species, respectively.

Netherlands Journal of Sea Research, 1994

C analysis of material from the shells of Arctica islandica supports the hypothesis that the clea... more C analysis of material from the shells of Arctica islandica supports the hypothesis that the clear and definite banding of these shells are of annual origin. The pulse of 14 C around 1960, resulting from atmospheric nuclear bomb testing was recorded in the shells at a location in concurrence with that expected from band counting. The observed cyclic variation in stable isotopes of oxygen and carbon coincides with growth bands. This variation, at least for 18 O, agrees with annual temperature variations. This suggests that growth bands are a reflection of seasonally determined differential growth rates. The longevity of this species, coupled with variations in increment width, may provide important information regarding growth and productivity, as well as a record of past environmental conditions.