Paleoceanography of sediment cores SO202-27-6 during MIS 3

The modern subarctic Pacific is characterized by a steep salinity-driven surface water stratification, which hampers the supply of saline and nutrient-rich deeper waters into the euphotic zone, limiting productivity. However, the strength of the halocline might have varied in the past. Here, we present diatom oxygen (d18Odiat) and silicon (d30Sidiat) stable isotope data from the open subarctic North-East (NE) Pacific (SO202-27-6; Gulf of Alaska), in combination with other proxy data (Neogloboquadrina pachydermasin d18O, biogenic opal, Ca and Fe intensities, IRD), to evaluate changes in surface water hydrography and productivity during Marine Isotope Stage (MIS) 3, characterized by millennial-scale temperature changes (Dansgaard-Oeschger (D-O) cycles) documented in Greenland ice cores.

Phosphorus in bottom sediments and micronodules from Core VITYAZ7414, Central Indian Ocean

Study of phosphorus distribution in grain size fractions of eupelagic clays showed high (up to 3%) content of P in Fe-Mn micronodules that can contain up to 20-30% of total P. Mineral P associated with Fe in ocean sediments is an analog of manganese in ocean sedimentogenesis. Sharp decrease of P contents in ocean Fe-Mn nodules compared to ones from seas results from decrease of Fe contents and partial neutralization of Fe activity by Mn.

Geomicrobiology, bulk sediment characteristics and pore water chemistry of sediment cores from Laguna Potrok Aike, southern Patagonia

Authigenic minerals can form in the water column and sediments of lakes, either abiotically or mediated by biological activity. Such minerals have been used as paleosalinity and paleoproductivity indicators and reflect trophic state and early diagenetic conditions. They are also considered potential indicators of past and perhaps ongoing microbial activity within sediments. Authigenic concretions, including vivianite, were described in late glacial sediments of Laguna Potrok Aike, a maar lake in southernmost Argentina. Occurrence of iron phosphate implies specific phosphorus sorption behavior and a reducing environment, with methane present. Because organic matter content in these sediments was generally low during glacial times, there must have been alternative sources of phosphorus and biogenic methane. Identifying these sources can help define past trophic state of the lake and diagenetic processes in the sediments. We used scanning electron microscopy, phosphorus speciation in bulk sediment, pore water analyses, in situ ATP measurements, microbial cell counts, and measurements of methane content and its carbon isotope composition (d13C CH4) to identify components of and processes in the sediment. The multiple approaches indicated that volcanic materials in the catchment are important suppliers of iron, sulfur and phosphorus. These elements influence primary productivity and play a role in microbial metabolism during early diagenesis. Authigenic processes led to the formation of pyrite framboids and revealed sulfate reduction. Anaerobic oxidation of methane and shifts in pore water ion concentration indicated microbial influence with depth. This study documents the presence of active microbes within the sediments and their relationship to changing environmental conditions. It also illustrates the substantial role played by microbes in the formation of Laguna Potrok Aike concretions. Thus, authigenic minerals can be used as biosignatures in these late Pleistocene maar sediments.

Phosphorus geochemistry of eastern equatorial Pacific Ocean sediments

We determined phosphorus (P) concentrations in Leg 138 sediment samples from Sites 844, 846, and 851, using a sequential extraction technique to identify the P associated with five sedimentary components. Total concentrations of P (sum of the five components) ranged from 4 to 35 µmol P/g sediment, with mean values relatively similar between the three sites (11, 14, and 12 for Sites 844,846, and 851, respectively). Authigenic/biogenic P was the most important component in terms of percentage of total P (about 75%), with iron-bound P (13%), adsorbed P (2%-9%), and organic P (4%) of secondary importance; detrital P was a minor P sink (1%) in these sediments. Profiles of adsorbed P and iron-bound P show decreasing concentrations with age, indicating that these components have been affected by diagenesis and reorganization of P. A peak in iron-bound P may reflect higher fluxes of hydrothermally derived Fe to eastern equatorial Pacific Ocean sediments from 11 to 8 Ma. Lower detrital P values for western Site 851 reflect a greater distance of this site from a terrigenous source area, compared to that of Sites 844 and 846. Phosphorus mass accumulation rates (P-MARs; units of µmol P/cm**2/k.y.) were calculated using total P concentrations (not including the minor and oceanically unreactive detrital P component) and sedimentation rates and dry-bulk densities averaged over time intervals of 0.5 m.y. P-MARs generally decrease from 17 Ma to the present. Eastern transect Sites 844 and 846 display a decrease in P-MARs from about 30 to 10 in the interval from 17 to 8 Ma, while western transect Site 851 is highly variable during this interval. P-MARs increase to about 45 and stay relatively high from 8 to 6 Ma, then decrease toward the present to some of the lowest values of the record (about 10). The general trend of high P-MARs at about 6 Ma and decreasing values toward the present is correlated with other geochemical and sedimentary trends through this interval and may reflect (1) a change in net sediment and P burial, (2) a reorganization of fluxes with no change of net burial, or (3) a combination of the two.

Diversity of planktonic foraminifer fauna found in 18 deep-sea cores from the North Atlantic

Greenland stadial/interstadial cycles are known to affect the North Atlantic's hydrography and overturning circulation and to cause ecological changes on land (e.g., vegetation). Hardly any information, directly expressed as diversity indices, however, exists on the impacts of these millennial-scale variations on the marine flora and fauna. We calculated three diversity indices (species richness, Shannon diversity index, Hurlbert's probability of interspecific encounter) for the planktonic foraminifer fauna found in 18 deep-sea cores covering a time span back to 60 ka. Clear differences in diversity response to the abrupt climate change can be observed and some records can be grouped accordingly. Core SO82-05 from the southern section of the subpolar gyre, the cores along the British margin and core MD04-2845 in the Bay of Biscay show two modes of diversity distribution, with reduced diversity (uneven fauna) during cold phases and the reverse (even fauna) during warm phases. Along the Iberian margin high species diversity prevailed throughout most of the glacial period. The exceptions were the Heinrich stadials when the fauna abruptly shifted from an even to an uneven or less even fauna. Diversity changes were often abrupt, but revealed a high resilience of the planktonic foraminifer faunas. The subtropical gyre waters seem to buffer the climatic effects of the Heinrich events and Greenland Stadials allowing for a quick recovery of the fauna after such an event. The current work clearly shows that planktonic foraminifer faunas quickly adapt to climate change, albeit with a reduced diversity.

Palaeoceanographic data of sediment core MD01-2416

We present an SiF4 separation line, coupled to a laser fluorination system, which allows for an efficient combined silica d18O and d30Si analysis (50 min per sample). The required sample weight of 1.5-2.0 mg allows for high-resolution isotope studies on biogenic opal. Besides analytical tests, the new instrumentation set-up was used to analyse two marine diatom fractions (>63 µm, 10-20 µm) with different diatom species compositions extracted from a Bølling/Allerød-Holocene core section [MD01-2416, North-West (NW) Pacific] to evaluate the palaeoceanographic significance of the diatom isotopic signals and to address isotopic effects related to contamination and species-related isotope effects (vital and environmental effects). While d30Si offsets between the two fractions were not discernible, supporting the absence of species-related silicon isotope effects, systematic offsets occur between the d18O records. Although small, these offsets point to species-related isotope effects, as bias by contamination can be discarded. The new records strengthen the palaeoceanographic history during the last deglaciation in the NW Pacific characterized by a sequence of events with varying surface water structure and biological productivity. With such palaeoceanographic evolution it becomes unlikely that the observed systematic d18O offsets signal seasonal temperature variability. This calls for reconsideration of vital effects, generally excluded to affect d18O measurements.