Analyses of ferromanganese micronodules from the central Arctic Ocean

In order to determine geochemical compositions of Late Cenozoic Arctic seawater, oxide fractions were chemically separated from 15 samples of hand-picked ferromanganese micronodules (50-300 mu m). The success of the chemical separation is indicated by the fact that >97% of the Sr in the oxide fraction is seawater-derived. Rare-earth element (REE) abundances of the Arctic micronodule oxide fractions are much lower than those of bulk Fe-Mn nodules from other ocean basins of the world (e.g., 33 vs. 145 ppm Nd), but the Arctic oxides are enriched in Ce relative to Nd (Ce-N/Nd-N=2.2+/-0.5) and have convex-upward, shale-normalized REE patterns (Nd-N/Gd-N=0.61+/-0.06, Gd-N/Yb-N = 1.5+/-0.2, Nd-N/Yb-N = 0.9+/-0.2), typical of other hydrogenous and diagenetic marine Fe-Mn-oxides. Bulk sediment samples from the central Arctic Ocean have REE abundances and patterns that are characteristic of those of post-Archean shale. Non-detrital fractions (calcite + oxide coatings) of Recent Arctic foraminifera have REE abundances and patterns similar to those of Recent foraminifera from the Atlantic Ocean. Electron microprobe analyses (n=178) of transition elements in 29 Arctic Fe-Mn micronodules from five different stratigraphic intervals of Late Cenozoic sediment indicate that oxide accretion occurred as a result of hydrogenetic and diagenetic processes close to the sediment-seawater interface. Transition element ratios suggest that no oxide accretion occurred during transitions from oxic to suboxic diagenetic conditions. Only K is correlated with Si and Al, and ratios of these elements suggest that they are associated with illite or phillipsite. Ca and Mg are correlated with Mn, which indicates variable substitution of these elements from seawater into the manganate phase. The geochemical characteristics of Arctic Fe-Mn micronodules indicate that the REEs of the oxide fractions were ultimately derived from seawater. However, because of minute contributions of Sr from siliciclastic detritus during diagenesis or during the chemical leaching procedure, Sr isotope compositions of the oxide fractions cannot be used to trace temporal changes in the Sr-87/Sr-86 ratio of Arctic seawater or to improve the chronostratigraphy.

Chemical and isotope compositions of sediments from DSDP Hole 86-576A and Core LL44-GPC-3

A record of changes in Pb and Sr isotopic composition of two cores (DSDP 86-576A and LL44- GPC-3) from the red clay region of the central North Pacific has been determined for the past 60-65 million years. The isotope records of the eolian silicate fraction of the red clays reflect the change in source area as the core sites migrated under different wind systems. The Sr isotope compositions of eolian silicate material are consistent with Asian loess and North American arc volcanism that has been recognized from mineralogical studies. The silicate-bound eolian Pb isotopic compositions similarly reflect Asian loess and arc volcanism. The isotope records of three ferromanganese crusts from similar locations in the central Pacific are similar to the eolian component of red clays, but offset to less radiogenic values. This may be due to two mechanisms: (1) Pb that can be removed from eolian material by seawater is much less radiogenic, or less likely (2) hydrothermal Pb can be transported further away from venting sites through particle exchange with seawater, despite hydrothermal venting acting as a net sink of oceanic Pb. The temporal changes in Pb isotopes in the ferromanganese crusts, bulk red clays and eolian silicates are similar although offset from each other suggesting that eolian deposition is an important source of Pb to seawater and to ferromanganese crusts. This contrasts with the Atlantic and Southern Ocean where more intense deep water flow leads to isotopic gradients in FeMn crusts that do not reflect surface water conditions immediately above the crust. A mechanism is proposed which accounts for Pacific deepwater Pb being isotopically influenced by eolian deposition.

Age determination and foraminiferal faunas in sediment core MD99-2275

A high-resolution study of palaeoceanographic changes off North Iceland during the time period 8600-5200 cal year BP is based on benthic and planktonic foraminiferal assemblages. The core material (MD99-2275) was obtained from about 440 m water depth on the eastern part of the North Icelandic shelf. Changes in the faunal composition are interpreted to be mainly caused by variations in the strength of the relatively warm, high-salinity Irminger Current and the cold East Icelandic Current, which have been shown to be linked to the climatic changes in the North Atlantic region. Environmental proxies at that site are particularly sensitive to palaeoceanographic changes due to its position close to the marine Polar Front. Benthic assemblages show that relatively cold conditions prevailed at the base of the record. An increase in the influence of Atlantic water masses at the sea floor is seen at around 8400 cal year BP, whereas the surface waters were relatively warm already at 8600 cal year BP. The warming was interrupted by a cold event at around 8100-8000 cal year BP, registered both in the bottom and surface waters and correlated with the so-called 8.2 kyr cooling event. Both the benthic and the planktonic faunal compositions indicate that the Irminger Current had maximum influence in the area between 8000 and about 7300 cal year BP, followed by a gradually decreasing influence through the remaining part of the studied time interval. It is suggested that the contribution of Atlantic water masses from the east and north-east to the Arctic Surface waters off North Iceland increased after around 7000 cal year BP, and that this was further intensified after 6200 cal year BP. At present, the Arctic Surface Water north of Iceland consists of Polar waters, intermittently with direct influence from the East Greenland Current, mixed with Atlantic waters derived from the eastern part of the Nordic Seas. A comparison of the mean values of selected environmental proxies in the interval 8600-5200 cal year BP with the upper part of the same core shows that the water masses north of Iceland were considerably warmer during the Holocene thermal maximum than during the last 2000 cal year. In general, results from core MD99-2275 are in accordance with other marine records from the North Icelandic shelf and the northern North Atlantic region, although a detailed comparison on a centennial time scale is hampered by problems with spatial as well as temporal changes in the marine reservoir ages in the region.