Stable isotopes of foraminifers, mineralogy and ages of sediment core GeoB1523-1

Today the western tropical Atlantic is the most important passage for cross-equatorial transfer of heat in the form of warm surface water flowing from the South into the North Atlantic. Circulation changes north of South America may thus have influenced the global thermohaline circulation system and high northern latitude climate. Here we reconstruct late Quaternary variations of western equatorial Atlantic surface circulation and Amazon lowland climate obtained from a multiproxy sediment record from Ceará Rise. Variations in the illite/smectite ratio suggest drier climatic conditions in the Amazon Basin during glacials relative to interglacials. The 230Thex-normalized fluxes and the 13C/12C record of organic carbon indicate that sea level fluctuations, shelf topography, and changes of the surface circulation pattern controlled variations and amplitude of terrigenous sediment supply to the Ceará Rise. We attribute variations in thermocline depth, reconstructed from vertical planktic foraminiferal oxygen isotope gradients and abundances of the phytoplankton species Florisphaera profunda, to changes in southeast trade wind intensity. Strong trade winds during ice volume maxima are associated with a deep western tropical Atlantic thermocline, strengthening of the North Brazil Current retroflection, and more vigorous eastward flow of surface waters.

Stable isotope record of DSDP Hole 81-552A in the northeastern Atlantic Ocean

Oxygen and carbon isotope ratios in benthic foraminifers have been determined at 10 cm intervals through the top 59 m of DSDP Hole 552A. This provides a glacial record of remarkable resolution for the late Pliocene and Pleistocene. The major glacial event which marked the onset of Pleistocene-like glacial-interglacial alternations was at about 2.4 m.y. ago. These very high-resolution data do not support the notion of significant Northern Hemisphere glaciation between 3.2 and 2.4 m.y. ago.

Sedimentology and stable isotope record of DSDP Hole 68-503B

Samples from the upper portion of a cyclic pelagic carbonate sediment sequence in Deep-Sea Drilling Project (DSDP) hole 503B (4.0°N, 95.6°W) are the first group to be analyzed for paleoceanographic and paleoclimatic proxy-indicators of ice volume, deep ocean and surface water circulation, and atmospheric circulation in order to resolve the complex origin of the cyclicity. Temporal resolution is taken from the delta18O time scale, most other parameters are calculated in terms of their mass flux to the seafloor. CaCO3 percent in the sediments fluctuates in the well-known Pacific pattern and is higher during glacial times. The fluxes of opal and organic carbon have patterns similar to each other and show a variability of a factor of 2.5 to 4. The longer organic carbon record shows flux maxima during both glacial and interglacial times. The accumulation patterns of both opal and organic carbon suggest that the variability in surface water productivity and/or seafloor preservation of those materials is not simply correlated to glacial or interglacial periods. Eolian dust fluxes are greater during interglacial periods by factors of 2 to 5, indicating that eolian source regions in central and northern South America were more arid during interglacial periods. The record of eolian grain size provides a semiquantitative estimation of the intensity of the transporting winds. The eolian data suggest more intense atmospheric circulation during interglacial periods, opposite to the anticipated results. We interpret this observation as recording the southerly shift of the intertropical convergence zone to the latitude of hole 503B during glaciations.

A reconstruction of atmospheric carbon dioxide and its stable carbon isotopic composition from the penultimate glacial maximum to the glacial inception

The reconstruction of the stable carbon isotope evolution in atmospheric CO2 (d13Catm ), as archived in Antarctic ice cores, bears the potential to disentangle the contributions of the different carbon cycle fluxes causing past CO2 variations. Here we present a new record of d13Catm before, during and after the Marine Isotope Stage 5.5 (155 000 to 105 000 years BP). The record was derived with a well established sublimation method using ice from the EPICA Dome C (EDC) and the Talos Dome ice cores in East Antarctica. We find a 0.4 permil shift to heavier values between the mean d13Catm level in the Penultimate (~ 140 000 years BP) and Last Glacial Maximum (~ 22 000 years BP), which can be explained by either (i) changes in the isotopic composition or (ii) intensity of the carbon input fluxes to the combined ocean/atmosphere carbon reservoir or (iii) by long-term peat buildup. Our isotopic data suggest that the carbon cycle evolution along Termination II and the subsequent interglacial was controlled by essentially the same processes as during the last 24 000 years, but with different phasing and magnitudes. Furthermore, a 5000 years lag in the CO2 decline relative to EDC temperatures is confirmed during the glacial inception at the end of MIS 5.5 (120 000 years BP). Based on our isotopic data this lag can be explained by terrestrial carbon release and carbonate compensation.

Age determination, planktic foraminifera and stable istopes of sediment core MSM05/5_712-2

A sediment core from the West Spitsbergen continental margin was studied to reconstruct climate and paleoceanographic variability during the last ~9 ka in the eastern Fram Strait. Our multiproxy evidence suggests that the establishment of the modern oceanographic configuration in the eastern Fram Strait occurred stepwise, in response to the postglacial sea-level rise and the related onset of modern sea-ice production on the shallow Siberian shelves. The late Early and Mid Holocene interval (9 to 5 ka) was generally characterized by relatively unstable conditions. High abundance of the subpolar planktic foraminifer species Turborotalita quinqueloba implies strong intensity of Atlantic Water (AW) inflow with high productivity and/or high AW temperatures, resulting in a strong heat flux to the Arctic. A series of short-lived cooling events (8.2, 6.9. and 6.1 ka) occurred superimposed on the warm late Early and Mid Holocene conditions. Our proxy data imply that simultaneous to the complete postglacial flooding of Arctic shallow shelves and the initiation of modern sea-ice production, strong advance of polar waters initiated modern oceanographic conditions in the eastern Fram Strait at ~5.2 ka. The Late Holocene was marked by the dominance of the polar planktic foraminifer species Neogloboquadrina pachyderma, a significant expansion of sea ice/icebergs, and strong stratification of the water column. Although planktic foraminiferal assemblages as well as sea surface and subsurface temperatures suggest a return of slightly strengthened advection of subsurface Atlantic Water after 3 ka, a relatively stable cold-water layer prevailed at the sea surface and the study site was probably located within the seasonally fluctuating marginal ice zone during the Neoglacial period.

Stable oxygen isotope record of Globigerinoides ruber of ODP Site 133-820 off the Great Barriere Reef

Oxygen-isotope ratio measurements are presented for the planktonic species Globigerinoides ruber collected from shallow-water, upper-slope sediments from Holes 820A and 820B in 280 m of water, on the seaward edge of the Great Barrier Reef. Correlation of the Site 820 isotope curve with deep-sea reference curves of the Pacific Ocean (Core V28-238, Hole 677A, Hole 607A) permits the definition of isotope stages 1 to 19 in the top 145 m of Holes 820A and 820B. However, paleontological data indicate that stages 4 and 7 might be missing and that two hiatuses occur at a depth of 8.05 to 12.1 and 34.55 to 35.8 mbsf. Using deep-sea Hole 677A as a reference for ice-volume variations, we determine the difference in isotopic signature between it and Site 820. We propose that this difference is a regional signal representing a progressive 4°C increase in surface-water temperature at Site 820. The proposed temperature change was initiated at about 400 k.y. and corresponds to a change from high-to-low frequency variations in Pleistocene isotope signals. We postulate that these changes may have catalyzed the growth of the Great Barrier Reef. The shift also coincides with changes in seismic character and some physical and chemical sediment characteristics.

Size-related stable isotope changes in Late Cretaceous and Recent planktic foraminifera from the western tropical Atlantic

The paleoecology of Cretaceous planktic foraminifera during the Late Cenomanian to Coniacian period (~95-86 Ma) remains controversial since much of the tropical marine record is preserved as chalk and limestone with uncertain geochemical overprints. Here we present delta13C and delta18O data from sieve size fractions of monospecific samples of exceptionally well preserved planktic foraminifera recovered during Ocean Drilling Program Leg 207 (Demerara Rise, western tropical Atlantic). Our results suggest that all species studied (Hedbergella delrioensis, Heterohelix globulosa, Marginotruncana sinuosa, Whiteinella baltica) grew primarily in surface waters and did not change their depth habitat substantially during their life cycle. Comparison of size-related ontogenetic trends in delta13C in Cretaceous and modern foraminifera further suggests that detection of dinoflagellate photosymbiosis using delta13C is confounded by physiological effects during the early stages of foraminifer growth, raising doubts about previous interpretations of photosymbiosis in small foraminifera species. We propose that obligate photosymbiosis involving dinoflagellates may not have evolved until the Campanian or Maastrichtian since our survey of Cenomanian-Coniacian species does not find the delta18O and delta13C size-related trends observed in modern foraminifer-dinoflagellate symbioses.