Sm-Nd isotope and Mn/Ca ratios for cleaned Neogloboquadrina pachyderma from ODP Hole 105-647A (Table 1)

The neodymium (Nd) isotope composition of ancient seawater is a potentially useful tracer of changes in continental inputs and ocean circulation on timescales of a few ka. Here we present the first Nd isotope record for seawater using sedimentary foraminifera cleaned using standard oxidative-reductive techniques. The data, along with Mn/Ca ratios, suggest that cleaned foraminifera provide a reliable record of Nd in seawater and hold out the prospect of using Nd in foraminifera to examine changes in seawater that accompany glacial-interglacial climatic cycles. The principal potential problem to be overcome with the use of forams as records of trace elements in ancient seawater is their diagenetic Fe-Mn coatings. These contain large amounts of Nd and other trace elements but can be cleaned off using highly reducing reagents. Mn(Ca ratios for the majority of the cleaned sedimentary foraminifera analysed here lie within the range (10-100 µmol/mol) that has yielded success in studies of transition elements in forams. Mass-balance modelling suggests that for residual Mn/Ca ratios <100 µmol/mol, Nd added to the foram in the coating will never shift the measured Nd isotope composition significantly away from the seawater value acquired by the foram test in the water column. Additionally, Nd concentrations measured in cleaned sedimentary foraminifera are comparable with those for a modern sample that has never encountered diagenetic fluids. Finally, core-top planktonic foraminifera for two sites have Nd isotope compositions that are identical to local surface seawater. The data we present here for Labrador Sea forams over the past 2.5 m.y. are interpreted in terms of changes in the seawater isotopic composition. The data show a pronounced shift from epsilon-Nd values of ~-12 to ~-19 in the period 2.5-1.5 Ma. This change is interpreted to result from the initiation of Northern Hemisphere glaciation and the increased derivation of Labrador Sea Nd via ice-rafting from Archaean terranes in central Canada. In combination with stable isotope and foraminiferal relative species abundance data, the new Nd data are consistent with the surface hydrography of the Labrador Sea being dominated by a fluctuating balance between cold, polar waters containing unradiogenic Nd and warm, subtropical waters containing more radiogenic Nd. The major change in Labrador Sea Nd that is observed in the past 2.5 Ma can, on its own, account for the change in the Nd isotope composition of North Atlantic Deep Water over the same time period.

(Table 1) Concentrations of CaCO3 and organic carbon, organic matter atomic C/N ratios and delta13C values of sediment samples from Site DSDP 594

CaCO3 and total organic carbon concentrations, organic matter C/N and carbon isotope ratios, and sediment accumulation rates in late Quaternary sediments from DSDP Site 594 provide information about glacial-interglacial variations in the delivery of organic matter to the Chatham Rise offshore of southeastern New Zealand. Low C/N ratios and nearly constant organic delta13C values of ?23? indicate that marine production dominates organic matter supply in both glacial and interglacial times during oxygen isotope stages 1 through 6 (0-140 ka) and 17 through 19 (660-790 ka). Increased organic carbon mass accumulation rates in isotope stages 2, 4, 6, and 18 record enhanced marine productivity during glacial maxima. Excursions of organic delta13C values to ca. ?29? in portions of isotope stage 2 suggest that the local concentration of dissolved CO2 was occasionally elevated during the last glacial maximum, probably as a result of short periods of lowered sea-surface temperature. Dilution of carbonates by clastic continental sediment generally increases at this location during glacial maxima, but enhanced delivery of land-derived organic matter does not accompany the increased accumulation of clastic sediments.

Stable oxygen isotope ratios of benthic and planktonic foraminifera from early Miocene sediments of DSDP Site 90-593 on the Challenger Plateau, Pacific Ocean (Table 1)

Oxygen isotope data are compared with relative abundances of selected planktic foraminifera through a ca. 15 m interval at DSDP Site 593 (Tasman Sea, southwest Pacific, 40°S) in which there are prominent changes in population sizes, as well as several evolutionary events. We focus on the relation between faunal and climatic histories. The base of early Miocene oxygen isotope Zone Mi1b (uppermost planktic foraminiferal Zone N.6) is identified from closesampled (c. 14 kyr) isotope records of Globigerina woodi and Cibicides kullenbergi. Chronostratigraphic interpolations, using the first occurrences of Globorotalia praescitula, G. mimea and Praeorbulina curva give an age estimate of ca. 18.4 Ma (cf. 18.1 -18.3 Ma for the base of the zone at DSDP Site 608 (type level, north Atlantic, 43°N) ). Another significant benthic delta18O enrichment event, informally designated as the base of zone "Mi1c", is identified 10 m higher in the sequence at ca. 17.8 Ma. Populations of Globoquadriau dehiscens and Globigerinoides trilobus (inferred to be near the southern margin of their distributions) either reduced considerably or withdrew, particularly in the vicinity of zone "Mi1c". A bioseries linking Globorotalia incognita with G. zealandica developed following the benthic delta18O enrichment spike at the base of Zone Mi1b; the latter species became extinct (at least regionally) just above the base of zone "Mi1c". In contrast, the apparently opportunistic Globorotlia praescitula increased dramatically in abundance at this time; there were also transformations in its architecture, leading to the evolutionary appearance of G. miozea. While planktic foraminifera abundances often do not closely covary with the detailed isotope records and tend to be more stable through time, the near coincidence of evolutionary and biogeographic events with isotopic events suggests at least indirect adaptive responses to climatic changes. Early Miocene middle-latitude planktic foraminiferal evolution, biogeography, and biostratigraphy, may be intimately connected with climatic history.

(Table 1) 87Sr/86Sr and Sr/Ca ratios in foraminiferal calcite from DSDP Holes 3-21 and 39-357

The 87Sr/86Sr ratio of ancient seawater, as recorded in marine carbonates, is an important tracer of long-term variations in ocean chemistry (Burke et al., 1982, doi:10.1130/0091-7613(1982)10<516:VOSSTP>2.0.CO;2; Peterman et al., 1970, doi:10.1016/0016-7037(70)90154-7; Dasch and Biscaye, 1971, doi:10.1016/0012-821X(71)90164-6; Veizer and Compston, 1974, doi:10.1016/0016-7037(74)90099-4; Brass, 1976, doi:10.1016/0016-7037(76)90025-9). However, the Sr isotope balance of the oceans has been difficult to constrain; consequently, attempts to evaluate the temporal 87Sr/86Sr changes have been largely qualitative. To constrain the causes of these variations we have measured 87Sr/86Sr ratios in carefully cleaned unrecrystallized foraminifera from DSDP sites 21 and 357. The data presented here have been quantitatively modelled taking advantage of recent advances in understanding of the Sr geochemical cycle. They suggest that whereas hydrothermal fluxes and carbonate recycling are of major importance in defining the marine 87Sr/86Sr ratio, the major control over its variations through the Cenozoic has been changes in the isotope composition of Sr derived from the weathering of silicate rocks.

Uvigerina spp. d18O and Mg/Ca measurements from sediment cores JR244-GC528 and MD07-3076

Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected d18O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer d13C and foraminifer/coral 14C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.