The nitrogen-isotope record preserved in Southern Ocean sediments, along with several geochemical tracers for the settling fluxes of biogenic matter, reveals patterns of past nutrient supply to phytoplankton and surface-water stratification in this oceanic region. Areal averaging of these spatial patterns indicates that reduction of the CO 2 'leak' from ocean to atmosphere by increased surface-water stratification south of the Polar Front made a greater contribution to the lowering of atmospheric CO 2 concentration during the Last Glacial Maximum than did the increased export of organic carbon from surface to deep waters occurring further north.The unstratified, nutrient-rich surface waters in the modern highlatitude ocean provide the main conduit for transferring deep-water CO 2 back into the atmosphere. This CO 2 'leak' to the atmosphere is particularly effective in the modern Southern Ocean, because of intensive vertical mixing and low nutrient utilization. Building on this observation, a series of papers 1-6 attributed the glacial lowering of atmospheric CO 2 either to enhanced biological removal of the nutrients and CO 2 in high-latitude surface waters resulting in higher sinking fluxes of organic matter (that is, higher export production), or to a lower supply rate of nutrients and CO 2 from intermediate waters produced by lower vertical mixing. But the extensive deepor intermediate-water anoxia predicted by these models, and the lack of palaeo-oceanographic evidence for increased export production in the glacial Southern Ocean, has led to the questioning of the validity of these models. Here we present new evidence which supports increased stratification south of the position of the modern Polar Front (MPF) during the Last Glacial Maximum as a mechanism that contributed to lower glacial atmospheric CO 2 and deep-water oxygen concentration.The sedimentary record of several palaeoproductivity proxies was recently presented as support for a large increase in the export flux of organic carbon in the Atlantic sector of the Southern Ocean, north of the position of the MPF, which could have contributed to lowering glacial atmospheric CO 2 as a result of Fe fertilization 7 . Here we combine a similar suite of geochemical proxies for palaeoproductivity and its fate with bulk-sediment d 15 N values to better constrain past changes in the nutrient balance of surface waters and its influence on atmospheric CO 2 . Bulk-sediment d 15 N provides a means of evaluating the fraction of surface nitrate utilized by phytoplankton [8][9][10][11] . By combining this information with export flux of nitrogen estimated from palaeoproductivity proxies, we constrain the supply rate of nitrate to surface waters. Our results indicate that, despite higher export flux of organic carbon in the Atlantic sector of the Southern Ocean north of the MPF during the last glacial period, the fraction of nitrate utilized by phytoplankton in this region did not increase, implying a sustained supply of nutrients (and thus CO 2 ) to surface waters. In...
The distribution of 234Th, 23øTh, and 228Th between dissolved and particulate forms was determined in 17 seawater samples from the Guatemala and Panama basins. Sampling was carried out in situ with battery-powered, submersible pumping systems in which the seawater first passed through a Nuclepore filter (1.0-/am pore size) and then through a cartridge packed with Nitex netting that was impregnated with MnO2 to scavenge the dissolved Th isotopes. Natural 234Th was used as the tracer for monitoring the efficiency of scavenging. For all three isotopes, most of the activity was found in the dissolved form. On the average 4% of the 234Th, 15% of the 228Th, and 17% of the 23øTh occurred in the particulate form, though the percentages were found to be strongly dependent on particle concentration. These distributions are not consistent with Chemical scavenging models that assume irreversible uptake of Th on particle surfaces. The results can be explained, however, if continuous exchange of Th isotopes between seawater and the particle surfaces is assumed. Vertical profiles of both particulate and dissolved 23øTh show increasing concentrations with depth, as required by the assumption of reversible exchange. Some of the dissolved 23øTh profiles, however, show a reversal of this trend near the bottom, indicating accelerated scavenging near the water/sediment interface. Kinetics of both adsorption and desorption can be examined if at least two Th isotopes are measured in the same samples. Results show that reaction times are short (a few months) compared to the residence time of suspended matter in the deep ocean (several years), indicating that particles suspended in the deep sea are close to equilibrium with respect to exchange of metals at their surfaces.
[1] There is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates. Notwithstanding its common incidence, this process (generally referred to as sediment focusing) is often difficult to recognize. If redistribution is near synchronous to deposition, the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked. Ignoring it, however, can lead to serious misinterpretations of sedimentary fluxes, particularly when past changes in export flux from the overlying water are inferred. In many instances, this problem can be resolved, at least for sediments deposited during the late Quaternary, by normalizing to the flux of 230 Th scavenged from seawater, which is nearly constant and equivalent to the known rate of production of 230 Th from the decay of dissolved 234 U. We review the principle, advantages and limitations of this method. Notwithstanding its limitations, it is clear that 230 Th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates.
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