Stable carbon isotopes of Planulina wuellerstorfi of sediment core MD01-2444

The observation that Greenland and Antarctic temperatures have followed a specific 'asymmetrical' pattern on millennial time-scales sets rigid constraints on any viable theory of abrupt climate change. The further observation that the very same asymmetry is also reflected in planktonic and benthic d18O measurements from the Northeast Atlantic has extended this constraint to include a specific response in the ocean. Here we present records of deep-water temperature, d18O and d13C variability from the Northeast Atlantic that help to shed light on the links between overturning circulation perturbations, sea-level variability and inter-hemispheric climate change on millennial time-scales. Results indicate that while deep-water temperatures in the Northeast Atlantic have tracked Greenland climate, the d18O signature of local deepwater (d18Odw) has varied in a manner more reminiscent of Antarctic temperature variability. The previously identified correspondence of Antarctic warm events with benthic d18O minima in the Northeast Atlantic is thus found to apply specifically to d18Odw minima, and to extend beyond Marine Isotope Stage 3 to the entirety of the last 50 ka. It is impossible to reconcile completely the Iberian Margin d18Odw record with existing reconstructions of millennial sea-level variability, leading to the conclusion that a significant portion of the d18Odw record must represent local hydrographic change. This is supported by benthic d13C measurements, which suggest the incursion during Greenland stadials of a colder, low-d18O and low-d13C water-mass, of presumed Antarctic origin. These observations confirm a one-to-one coupling of inter-hemispheric climate events with changes in the Atlantic overturning circulation, but fail to rule in or out a unique mechanism by which they were triggered.

Concentrations and isotopic ratios of thorium and uranium in pelagic sediments

The predictable in situ production of 230Th from the decay of uranium in seawater, and its subsequent removal by scavenging onto falling particles, provides a valuable tool for normalizing fluxes to the seafloor. We describe a new application, determination of the 232Th that dissolves in the water column and is removed to the seafloor. 232Th is supplied to the ocean in continental minerals, dissolution of which leads to a measurable standing stock in the water column. Sedimentary adsorbed 232Th/230Th ratios have the potential to provide a proxy for estimating the amount of dissolved material that enters the ocean, both today and in the past. Ten core top samples were treated with up to eight different leaching techniques in order to determine the best method for the separating adsorbed from lattice bound thorium. In addition, separate components of the sediments were analyzed to test whether clay dissolution was an important contribution to the final measurement. There was no systematic correlation between the strength of acid used in the leach and the measured 232Th/230Th ratios. In all cases clean foraminifera produced the same ratio as leaches on bulk sediment. In three out of five samples leaches performed on non-carbonate detritus in the <63 µm size fraction were also identical. Without additional water column data it is not yet clear whether there is a simple one to one correlation between the expected deep-water 232Th/230Th and that produced by leaching, especially in carbonate-rich sediments. However, higher ratios, and associated high 232Th adsorbed fluxes, were observed in areas with high expected detrital inputs. The adsorbed fraction was ~35-50% of the total 232Th in seven out of ten samples. Our 230Th normalized 232Th fluxes are reasonable by comparison to global estimates of detrital inputs to the ocean. In nine cases out of ten, the total 230Th-normalized 232Th flux is greater than predicted from the annual dust fall at each specific location, but lower than the average global detrital input from all sources.

Concentrations and accumulation rates of platinum-group elements and rhenium in Pacific and Atlantic sediments, DSDP and ODP data

The nature of Re-platinum-group element (PGE; Pt, Pd, Ir, Os, Ru) transport in the marine environment was investigated by means of marine sediments at and across the Cretaceous-Tertiary boundary (KTB) at two hemipelagic sites in Europe and two pelagic sites in the North and South Pacific. A traverse across the KTB in the South Pacific pelagic clay core found elevated levels of Re, Pt, Ir, Os, and Ru, each of which is approximately symmetrically distributed over a distance of ~1.8 m across the KTB. The Re-PGE abundance patterns are fractionated from chondritic relative abundances: Ru, Pt, Pd, and Re contents are slightly subchondritic relative to Ir, and Os is depleted by ~95% relative to chondritic Ir proportions. A similar depletion in Os (~90%) was found in a sample of the pelagic KTB in the North Pacific, but it is enriched in Ru, Pt, Pd, and Re relative to Ir. The two hemipelagic KTB clays have near-chondritic abundance patterns. The ~1.8-m-wide Re-PGE peak in the pelagic South Pacific section cannot be reconciled with the fallout of a single impactor, indicating that postdepositional redistribution has occurred. The elemental profiles appear to fit diffusion profiles, although bioturbation could have also played a role. If diffusion had occurred over ~65 Ma, the effective diffusivities are ~10**?13 cm**2/s, much smaller than that of soluble cations in pore waters (~10**?6 cm**2/s). The coupling of Re and the PGEs during redistribution indicates that postdepositional processes did not significantly fractionate their relative abundances. If redistribution was caused by diffusion, then the effective diffusivities are the same. Fractionation of Os from Ir during the KTB interval must therefore have occurred during aqueous transport in the marine environment. Distinctly subchondritic Os/Ir ratios throughout the Cenozoic in the South Pacific core further suggest that fractionation of Os from Ir in the marine environment is a general process throughout geologic time because most of the inputs of Os and Ir into the ocean have Os/Ir ratios >/=1. Mass balance calculations show that Os and Re burial fluxes in pelagic sediments account for only a small fraction of the riverine Os (<10%) and Re (<0.1%) inputs into the oceans. In contrast, burial of Ir in pelagic sediments is similar to the riverine Ir input, indicating that pelagic sediments are a much larger repository for Ir than for Os and Re. If all of the missing Os and Re is assumed to reside in anoxic sediments in oceanic margins, the calculated burial fluxes in anoxic sediments are similar to observed burial fluxes. However, putting all of the missing Os and Re into estuarine sediments would require high concentrations to balance the riverine input and would also fail to explain the depletion of Os at pelagic KTB sites, where at most ~25% of the K-T impactor's Os could have passed through estuaries. If Os is preferentially sequestered in anoxic marine environments, it follows that the Os/Ir ratio of pelagic sediments should be sensitive to changes in the rates of anoxic sediment deposition. There is thus a clear fractionation of Os and Re from Ir in precipitation out of sea water in pelagic sections. Accordingly, it is inferred here that Re and Os are removed from sea water in anoxic marine depositional regimes.