B-isotope and Mg/Ca ratios of planktonic foraminifera from ODP Hole 108-668B and esimates of salinity, alkalinity and pCO2 (Table 1)

Knowledge of past atmospheric pCO2 is important for evaluating the role of greenhouse gases in climate forcing. Ice core records show the tight correlation between climate change and pCO2, but records are limited to the past ~900 kyr. We present surface ocean pH and pCO2 data, reconstructed from boron isotopes in planktonic foraminifera over two full glacial cycles (0-140 and 300-420 kyr). The data co-vary strongly with the Vostok pCO2-record and demonstrate that the coupling between surface ocean chemistry and the atmosphere is recorded in marine archives, allowing for quantitative estimation of atmospheric pCO2 beyond the reach of ice cores.

Variability of the Brazil Current during the late Holocene analysed on two marine sediment cores

Our understanding of the centennial-scale variability of the Brazil Current (BC) during the late Holocene is elusive because of the lack of appropriate records. Here we used the Mg/Ca and oxygen isotopic composition of planktonic foraminifera from two marine sediment cores collected at 27° S and 33° S off southeastern South America to assess the late Holocene variability in the upper water column of the BC. Our results show in phase fluctuations of up to 3 °C in sea surface temperatures (SST), and 0.8 per mil in oxygen isotopic composition of surface sea water, a proxy for relative sea surface salinity (SSS). Time-series analyses of our records indicate a cyclicity with a period of ca. 730 yr. We suggest that the observed cyclicity reflects variability in the strength of the BC associated to changes in the Atlantic meridional overturning circulation (AMOC). Positive (negative) SST and SSS anomalies are related to a strong (weak) BC and a weak (strong) AMOC. Moreover, periods of peak strength in the BC occur synchronously to a weak North Brazil Current, negative SST anomalies in the high latitudes of the North Atlantic, and positive (negative) precipitation anomalies over southeastern South America (equatorial Africa), further corroborating our hypothesis. This study shows a tight coupling between the variability of the BC and the high latitudes of the North Atlantic mediated by the AMOC even under late Holocene boundary conditions.

Age determinations of sediment cores from the Baffin Shelf

Cores HU82-034-057 and HU84-035-008, Resolution Basin, SE Baffin Shelf, contain 200 and 450 cm, respectively, of sediment that spans the Younger Dryas chron. In both cores the interval is bracketed by 14C dates on foraminifera or molluscs. These sites were close to the margin of the late Wisconsin (Foxe) ice sheet as it flowed toward the Labrador Sea. Prior to 11 ka, both cores record moderate to high accumulations of foraminifera, relatively high del 18O values in planktonic foraminifera, and low values of detrital carbonate. The diatom and percent opal records imply occasional seasonally open water conditions. During part of the Younger Dryas chron both the diatom and opal analyses imply a shutoff of biogenic silica production, suggesting surface water conditions affected by increased sea ice and/or reduced nutrients. In addition, the Younger Dryas interval is marked by an increase in coarse sand and detrital carbonate, a decrease in total organic carbon and foraminifera, and high rates of sediment accumulation. The inferred environment during the Younger Dryas is ice-proximal. In HU82-034-057, the foraminifera and other data suggest a change in conditions during the middle part of the Younger Dryas chron.

Chemistry of interstitial waters of ODP Leg 112 samples

Two distinct hydrogeochemical regimes currently dominate the Peruvian continental margin. One, in shallower water (150-450 m) shelf to upper-slope regions, is characterized by interstitial waters with strong positive chloride gradients with depth. The maximum measured value of 1043 mM chloride at Site 680 at ITS corresponds to a degree of seawater evaporation of ~2 times. Major ion chemistry and strontioum isotopic composition of the interstitial waters suggest that a subsurface brine that has a marine origin and is of pre-early Miocene "age," profoundly influences the chemistry and diagenesis of this shelf environment. Site 684 at ~9°S must be closest to the source of this brine, which becomes diluted with seawater and/or interstitial water as it flows southward toward Site 686 at ~13?S (and probably beyond) at a rate of approximately 3 to 4 cm/yr, since early Miocene time. The other regime, in deep water (3000-5000 m) middle to lower-slope regions, is characterized by interstitial waters with steep negative and nonsteady-state chloride gradients with depth. The minimum measured value of 454 mM chloride, at Site 683 at ITS, corresponds to ~20% dilution of seawater chloride The most probably sources of these low-chloride fluids are gas hydrate dissociation and mineral (particularly clay) dehydration reactions. Fluid advection is consistent with (1) the extent of dilution shown in the chloride profiles, (2) the striking nonsteady-state depth profiles of chlorides at Sites 683 and 688 and of 87Sr/86Sr ratios at Site 685, and (3) the temperatures resulting from an average geothermal gradient of 50°C/km and required for clay mineral dehydration reactions. Strontium isotope data reveal two separate fluid regimes in this slope region: a more northerly one at Sites 683 and 685 that is influenced by fluids with a radiogenic continental strontium signature, and a southerly one at Sites 682 and 688 that is influenced by fluids with a nonradiogenic oceanic signatures. Stratigraphically controlled fluid migration seems to prevail in this margin. Because of its special tectonic setting, Site 679 at ITS is geochemically distinct. The interstitial waters are characterized by seawater chloride concentrations to -200 mbsf and deeper by a significantly lower chloride concentration of about two-thirds of the value in seawater, suggesting mixing with a meteoric water source. Regardless of the hydrogeochemical regime, the chemistry and isotopic compositions of the interstitial waters at all sites are markedly modified by diagenesis, particularly by calcite and dolomite crystallization.