Gerald Ganssen

Past temperature and d18O of surface ocean waters inferred from foraminiferal Mg/Ca ratios.

Determining the past record of temperature and salinity of ocean surface waters is essential for understanding past changes in climate, such as those which occur across glacial–interglacial transitions. As a useful proxy, the oxygen isotope composition (δ18O) of calcite from planktonic foraminifera has been shown to reflect both surface temperature and seawater δ18O, itself an indicator of global ice volume and salinity. In addition, magnesium/calcium (Mg/Ca) ratios in foraminiferal calcite show a temperature dependence due to the partitioning of Mg during calcification. Here we demonstrate, in a field-based calibration experiment, that the variation of Mg/Ca ratios with temperature is similar for eight species of planktonic foraminifera (when accounting for Mg dissolution effects). Using a multi-species record from the Last Glacial Maximum in the North Atlantic Ocean we found that past temperatures reconstructed from Mg/Ca ratios followed the two other palaeotemperature proxies: faunal abundance and alkenone saturation. Moreover, combining Mg/Ca and δ18O data from the same faunal assemblage, we show that reconstructed surface water δ18O from all foraminiferal species record the same glacial–interglacial change—representing changing hydrography and global ice volume. This reinforces the potential of this combined technique in probing past ocean–climate interactions.

Changes in east Atlantic deepwater circulation over the last 30,000 years: eight time slice reconstructions

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 1800 m about 13,500 14C years ago (mode 3). The formation of NADW recommenced abruptly north of Iceland 12,800–12,500 years ago and reached a volume approaching that of the Holocene, in the Younger Dryas (10,800–10,350 years B.P.). Another short-term shutdown of deepwater formation followed between 10,200 and 9,600 years B.P., linked to a further major meltwater pulse into the Atlantic. Each renewal of deepwater formation led to a marked release of fossil CO2 from the ocean, the likely cause of the contemporaneous 14C plateaus. Over the last 9000 years, deepwater circulation varied little from today, apart from a slight increase in AABW about 7000 14C years ago. It is also shown that the oxygenated Mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30,000 years.

Vigorous exchange between the Indian and Atlantic oceans at the end of the past five glacial periods

The magnitude of heat and salt transfer between the Indian and Atlantic oceans through ‘Agulhas leakage’ is considered important for balancing the global thermohaline circulation. Increases or reductions of this leakage lead to strengthening or weakening of the Atlantic meridional overturning and associated variation of North Atlantic Deep Water formation. Here we show that modern Agulhas waters, which migrate into the south Atlantic Ocean in the form of an Agulhas ring, contain a characteristic assemblage of planktic foraminifera. We use this assemblage as a modern analogue to investigate the Agulhas leakage history over the past 550,000 years from a sediment record in the Cape basin. Our reconstruction indicates that Indian–Atlantic water exchange was highly variable: enhanced during present and past interglacials and largely reduced during glacial intervals. Coherent variability of Agulhas leakage with northern summer insolation suggests a teleconnection to the monsoon system. The onset of increased Agulhas leakage during late glacial conditions took place when glacial ice volume was maximal, suggesting a crucial role for Agulhas leakage in glacial terminations, timing of interhemispheric climate change and the resulting resumption of the Atlantic meridional overturning circulation.

Sea surface temperature and productivity records for the past 240 kyr in the Arabian Sea

Abstract Deep-sea sediments of two cores from the western (TY93-929/P) and the southeastern (MD900963) Arabian Sea were used to study the variations of the Indian monsoon during previous climatic cycles. Core TY93-929/P was located between the SW monsoon driven upwelling centres off Somalia and Oman, which are characterized by large seasonal sea surface temperature (SST) and particle flux changes. By contrast, core MD900963, was situated near the Maldives platform, an equatorial ocean site with a rather small SST seasonality (less than 2°C). For both cores we have reconstructed SST variations by means of the unsaturation ratio of C37 alkenones, which is compared with the δ18O records established on planktonic foraminifera. In general, the SST records follow the δ18O variations, with an SST maximum during oxygen isotope stage 5.5 (the Last Interglacial at about 120–130 kyr) and a broad SST minimum during isotope stage 4 and 3.3 (approximately 40–50 kyr). The SST difference between the Holocene and the Last Glacial Maximum (LGM) is of the order of 2°C. In both cores the SSTs during isotope stage 6 are distinctly higher by 1–2°C than the cold SST minima during the last glacial cycle (LGM and stage 3). To reconstruct qualitatively the past productivity variations for the two cores, we used the concentrations and fluxes of alkenones and organic carbon, together with a productivity index based on coccolith species (Florisphaera profunda relative abundance). Within each core, there is a general agreement between the different palaeoproductivity proxies. In the southeastern Arabian Sea (core MD900963), glacial stages correspond to relatively high productivity, whereas warm interstadials coincide with low productivity. All time series of productivity proxies are dominated by a cyclicity of about 21–23 kyr, which corresponds to the insolation precessional cycle. A hypothesis could be that the NE monsoon winds were stronger during the glacial stages, which induced deepening of the surface mixed layer and injection of nutrients to the euphotic zone. By contrast, the records are more complicated in the upwelling region of the western Arabian Sea (core TY93-929/P). This is partly due to large changes in the sedimentation rates, which were higher during specific periods (isotope stages 6, 5.4, 5.2, 3 and 2). Unlike core MD900963, no simple relationship emerges from the comparison between the δ18O stratigraphy and productivity records. The greater complexity observed for core TY93-929/P could be the result of the superimposition of different patterns of productivity fluctuations for the two monsoon seasons, the SW monsoon being enhanced during interglacial periods, whereas the NE monsoon was increased during glacial intervals. A similar line of reasoning also could help explain the SST records by the superimposition of variations of three components: global atmospheric temperature, and SW and NE monsoon dynamics.

The effect of upwelling on the distribution and stable isotope composition of Globigerina bulloides and Globigrinoides ruber (planktic foraminifera) in modern surface waters of the NW Arabian Sea.

Abstract Hydrographic changes in the NW Arabian Sea are mainly controlled by the monsoon system. This results in a strong seasonal and vertical gradient in surface water properties, such as temperature, nutrients, carbonate chemistry and the isotopic composition of dissolved inorganic carbon ( δ 13 C DIC ). Living specimens of the planktic foraminifer species Globigerina bulloides and Globigerinoides ruber , were collected using depth stratified plankton tows during the SW monsoon upwelling period in August 1992 and the NE monsoon non-upwelling period in March 1993. We compare their distribution and the stable isotope composition to the seawater properties of the two contrasting monsoon seasons. The oxygen isotope composition of the shells ( δ 18 O shell ) and vertical shell concentration profiles indicate that the depth habitat for both species is shallower during upwelling (SW monsoon period) than during non-upwelling (NE monsoon period). The calcification temperatures suggest that most of the calcite is precipitated at a depth level just below the deep chlorophyll maximum (DCM), however above the main thermocline. Consequently, the average calcification temperature of G. ruber and G. bulloides is lower than the sea surface temperature by 1.7±0.8 and 1.3±0.9 °C, respectively. The carbon isotope composition of the shells ( δ 13 C shell ) of both species differs from the in situ δ 13 C DIC found at the calcification depths of the specimens. The observed offset between the δ 13 C shell and the ambient δ 13 C DIC results from (1) metabolic/ontogenetic effects, (2) the carbonate chemistry of the seawater and, for symbiotic G. ruber , (3) the possible effect of symbionts or symbiont activity. Ontogenetic effects produce size trends in Δ δ 13 C shell–DIC and Δ δ 18 O shell–w : large shells of G. bulloides (250–355μm) are 0.33‰ ( δ 13 C) and 0.23‰ ( δ 18 O) higher compared to smaller ones (150–250 μm). For G. ruber , this is 0.39‰ ( δ 13 C) and 0.17‰ ( δ 18 O). Our field study shows that the δ 13 C shell decreases as a result of lower δ 13 C DIC values in upwelled waters, while the effects of the carbonate system and/or temperature act in an opposite direction and increase the δ 13 C shell as a result lower [CO 3 2− ] (or pH) values and/or lower temperature. The Δ δ 13 C shell–DIC [CO 3 2− ] slopes from our field data are close to those reported literature from laboratory culture experiments. Since seawater carbonate chemistry affects the δ 13 C shell in an opposite sense, and often with a larger magnitude, than the change related to productivity (i.e. δ 13 C DIC ), higher δ 13 C shell values may be expected during periods of upwelling.

The isotopic signature of planktonic foraminifera from NE Atlantic surface sediments: implications for the reconstruction of past oceanic conditions

The stable isotope compositions of the planktonic foraminifera Globigerina bulloides, Globigerinoides ruber (white and pink varieties), Globigerinoides trilobus, Globorotalia inflata and Globorotalia truncatulinoides (right‐ and left‐coiling types) were examined as recorders of North Atlantic surface water properties based on 40 box‐core surface sediments between 60° and 30°N. While G. ruber (white and pink varieties) and G. trilobus mainly reflect summer surface water conditions in their oxygen isotope composition, G. bulloides reflects temperatures of the northward‐migrating spring bloom, February–March in the south to May–June in the north. Our data show that G. bulloides cannot be regarded as an indicator for summer temperatures as deduced from Duplessy et al.’s data. Gt. inflata and Gt. truncatulinoides (right‐ and left‐coiling) build their shells in the coldest waters compared with the other species and reflect temperatures between 100 and 400 m water depth. The difference in oxygen isotope composition between G. bulloides and G. inflata serves as a proxy for water mass stratification. G. bulloides is the only species that gives a distinct pattern in its carbon isotopic composition showing a high correlation with the surface water phosphate values along the transect and may serve as a proxy for palaeonutrients and/or productivity.

Origin and Significance of Isotope Shifts in Pennsylvanian Carbonates (Asturias, NW Spain)

ABSTRACT The primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water. An inadequate understanding of this variability, as well as different diagenetic environments, severely limit the interpretation of the stable-isotope record of shoalwater carbonates. In order to investigate this primary and diagenetic variability along a Bashkirian-Moscovian platform-to-basin transect, 13C and 18O analyses have been performed on more than 1000 matrix micrite, carbonate cement, and brachiopod shell samples. In isotope analysis, these different carbonate materials tend to complement each other, inasmuch as they have different advantages and shortcomings. The resulting data reveal spatial trends in 13C and 18O signatures from platform top (lower values) to basin (higher values). In the case of 13C from pristine brachiopods, this trend can be explained by the long residence time (aging) of platform-top water masses. In the case of brachiopod 18O, this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break. Micrite and marine cement isotopic values from the platform interior were reset (lowered) during pervasive early meteoric diagenesis. In contrast, micrite and marine cement isotopic values from the outer platform, slope, and basin show higher values close to the assumed Pennsylvanian seawater isotopic composition. This implies that isotopic data from shoalwater carbonates (including pristine brachiopod shells) might not necessarily reflect paleoceanographic trends of the open-ocean water masses because of changes in coastal water-mass isotope signature and interaction with early meteoric fluids.

Stable isotope 'vital effects' in coccolith calcite

Uncertainties about the origin of the many disequilibrium or ‘vital effects’ in a variety of calcifying organisms, and whether these effects are constant or variable, have hampered paleoceanographic application of carbon and oxygen isotopic ratios. Unraveling the source of these effects will improve paleoceanographic applications and may provide new information on changes in cell physiology and ecology. Culture of eight species of coccolithophorids, a dominant marine phytoplankton group, reveals a 5‰ array of disequilibrium or ‘vital effects’ in both the carbon and oxygen isotopic composition of coccolith calcite. In moderate light and nutrient-replete cultures, oxygen isotopic fractionation and carbon isotopic fractionation correlates directly with cell division rates and correlates inversely with cell size across a range of species. However, when growth rates of a single species are increased or decreased by higher or lower light levels, ϵ18O is relatively invariant. Likewise, growth rate variations as a function of temperature do not influence coccolith ϵ18O; the slope of the ϵ18O vs. temperature relation in cultures of both Gephyrocapsa oceanica and Helicosphaera carteri is the same as for abiogenic carbonates. This suggests a constant, species-specific isotopic fractionation which does not vary with cell physiology. The constancy of vital effects suggests that coccolith stable isotopes will provide reliable phase for paleoceanographic reconstruction of temperature and seawater chemistry, as long as monospecific fractions are analyzed or changes in nannofossil assemblages are accounted for with species-specific correction factors. We suspect that the cell size, and its constraints on the rate of CO2 diffusion relative to C fixation, may be the first order influence on coccolith stable isotope vital effects. A quantitative model of this process may provide important constraints on mechanisms of carbon acquisition of coccolithophorids in both modern and extinct species.

Northern Indian Ocean upwelling cells and the stable isotope composition of living planktonic foraminifers

Oxygen and carbon isotope ratios were determined of the shells of living planktonic foraminifers, collected along a west-east transect in northern Indian Ocean surface waters. The δ18O values of all species do not correspond closely to the temperature fluctuations caused by the upwelling of colder subsurface waters. This is explained mainly by rapid warming of the episodically advected colder subsurface waters into the euphotic zone. The carbon isotope composition of shells of the non-spinose Neoqloboquadrina dutertrei, Globorotalia menardii and to a lesser extent Globigerinita glutinata clearly shows depletion in 13C within upwelling areas. The δ13C values of the symbiont-bearing, spinose carnivorous species Globigerinoides trilobus, Globigerinoides ruber and Globigerinella siphonifera only exhibit minor variations. Globigerina bulloides shows an enrichment in 13C. We explained the observed δ13C variations assuming that the species represent different phases of the pulsating upwelling system; N. dutertrei and G. menardii representing its initial phase, when nutrient-rich, 13C-depleted water reaches the surface. G. bulloides invades later, after the phyto- and zooplankton blooms are well developed and the uptake of 12C in the organic material has balanced or even exceeded the amount of 12C carried into the surface waters through upwelling. Thus the 13C/12C ratios of the various species depend on their timing of optimum occurrence determined by the intensity, longevity and corresponding biological activities in the upwelling cycle. Unknown vital effects caused by food source and/or metabolic activities may further influence the 13C/12C ratios of the various species.

Oxygen isotope/salinity relationship in the Northern Indian Ocean

We analyze the surface δ18O-salinity relationships of the Bay of Bengal and the Arabian Sea, in the northern Indian Ocean, known for their contrasting hydrological conditions. New measurements of these tracers show a very low δ18O-salinity slope associated with the strong dilution in the Bay of Bengal, but a slope more typical of this latitude in the Arabian Sea. Although this region is marked by a complex monsoonal regime, numerical modeling using a box model and a general circulation model is able to capture the δ18O-salinity slope and its geographical variation. Both models clearly show that the low δ18O-salinity slope is due to the evaporation-minus-precipitation balance, with an important contribution of the continental runoff in the Bay of Bengal. Although the low value of these slopes (∼0.25) makes past salinity reconstructions uncertain, insight into the Last Glacial Maximum conditions shows a probable stability of these slopes and limited error on paleosalinity.