Geochemical and Nd isotopic investigations of sediments from the South China Sea

Secular variations in geochemistry and Nd isotopic data have been documented in sediment samples at ODP Site 1148 in the South China Sea. Major and trace elements show significant changes at ca. 29.5 Ma and 26-23 Ma, whereas epsilon-Nd values show a single change at ca. 26-23 Ma. Increases in Al/Ti, Al/K, Rb/Sr, and La/Lu ratios and a decrease in the Th/La ratio of the sediments beginning at 29.5 Ma are consistent with more intense chemical weathering in the source region. The abrupt change in Nd isotopes and geochemistry at ca. 26-23 Ma coincides with a major discontinuity in the sedimentology and physical properties of the sediments, implying a drastic change in sedimentary provenance and environment at the drill site. Comparison of the Nd isotopes of sediments from major rivers flowing into the South China Sea suggests that pre-27 Ma sediments were dominantly derived from a southwestern provenance (Indochina-Sunda Shelf and possibly northwestern Borneo), whereas post-23 Ma sediments were derived from a northern provenance (South China). This change in provenance from southwest to north was largely caused by ridge jumping during seafloor spreading of the South China Sea, associated with a southwestward expansion of the ocean basin crust and a global rise in sea level. Thus, the geochemical and Nd isotopic changes in the sediments at ODP Site 1148 are interpreted as a response to a major plate reorganization in SE Asia at ca. 25 Ma.

Siliceous phytoplankton in middle Eocene sediments of ODP Hole 207-1260A

The Middle Eocene diatom and silicoflagellate record of ODP Site 1260A (Demerara Rise) is studied quantitatively in order to throw light on the changes that siliceous phytoplankton communities experienced during a Middle Eocene warming event that occurred between 44.0 and 42.0 Ma. Both Pianka's overlap index, calculated per couple of successive samples, and cluster analysis, point to a number of significant turnover events highlighted by changes in the structure of floristic communities. The pre-warming flora, dominated by cosmopolitan species of the diatom genus Triceratium, is replaced during the warming interval by a new and more diverse assemblage, dominated by Paralia sulcata (an indicator of high productivity) and two endemic tropical species of the genus Hemiaulus. The critical warming interval is characterized by a steady increase in biogenic silica and a comparable increase in excess Ba, both reflecting an increase in productivity. In general, it appears that high productivity not only increased the flux of biogenic silica, but also sustained a higher diversity in the siliceous phytoplankton communities. The microflora preserved above the critical interval is once again of low diversity and dominated by various species of the diatom genus Hemiaulus. All assemblages in the studied material are characterized by the total absence of continental and benthic diatoms and the relative abundance of neritic forms, suggesting a transitional depositional environment between the neritic and the oceanic realms.

Trace element ratios of Globigerina bulloides from the southwest Pacific Ocean

We present Mg/Ca data for Globigerina bulloides from 10 core top sites in the southwest Pacific Ocean analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Mg/Ca values in G. bulloides correlate with observed ocean temperatures (7°C-19°C), and when combined with previously published data, an integrated Mg/Ca-temperature calibration for 7°C-31°C is derived where Mg/Ca (mmol/mol) = 0.955 * e**(0.068 * T) (r**2 = 0.95). Significant variability of Mg/Ca values (20%-30%) was found for the four visible chambers of G. bulloides, with the final chamber consistently recording the lowest Mg/Ca and is interpreted, in part, to reflect changes in the depth habitat with ontogeny. Incipient and variable dissolution of the thin and fragile final chamber, and outermost layer concomitantly added to all chambers, caused by different cleaning techniques prior to solution-based ICPMS analyses, may explain the minor differences in previously published Mg/Ca-temperature calibrations for this species. If the lower Mg/Ca of the final chamber reflects changes in depth habitat, then LA-ICPMS of the penultimate (or older) chambers will most sensitively record past changes in near-surface ocean temperatures. Mean size-normalized G. bulloides test weights correlate negatively with ocean temperature (T = 31.8 * e**(-30.5*wtN); r**2 = 0.90), suggesting that in the southwest Pacific Ocean, temperature is a prominent control on shell weight in addition to carbonate ion levels.

Rare earth elements from the Atlantic Sector

We present a Rare Earth Elements (REE) record at decadal resolution determined in the EPICA ice core drilled in Dronning Maud Land (EDML) in the Atlantic Sector of the East Antarctic Plateau, covering the transition from the last glacial age (LGA) to the early Holocene (26 600-7500 yr BP). Additionally, samples from potential source areas (PSAs) for Antarctic dust were analysed for their REE characteristics. The dust provenance is discussed by comparing the REE fingerprints in the ice core and the PSAs samples. We find a shift in REE composition at 15 200 yr BP in the ice core samples. Before 15 200 yr BP, the dust composition is very uniform and its provenance was likely to be dominated by a South American source. After 15 200 yr BP, multiple sources such as Australia and New Zealand become relatively more important, albeit South America is possibly still an important dust supplier. A similar change in the dust characteristics was observed in the EPICA Dome C ice core at around ~15 000 yr BP. A return to more glacial dust characteristics between ~8300 and ~7500 yr BP, as observed in the EPICA Dome C core, could not be observed in the EDML core. Consequently, the dust provenance at the two sites must have been different at that time.

(Table 1, page 434-435) Bulk chemical composition of S.W. Pacific island arc Mn-oxide crusts

Hydrothermal Mn-oxide crusts have been removed from the Tonga-Kermadec Ridge, the first such hydrothermal deposits to be reported in the S.W. Pacific island arc. In several respects the deposits are similar to hydrothermal Mn-crusts from oceanic spreading centre settings. They are limited in areal extent, comprise well-crystalline birnessite and generally display extreme fractionation of Mn from Fe. They are strongly depleted in many elements compared to hydrogenous Mn deposits but are comparatively enriched in Li, Zn, Mo and Cd. The Group IA and Group IIA metals show strong intercorrelations and the behaviour of Mg in the purest samples may indicate the extent to which normal seawater has influenced the composition of the deposits. Certain aspects of the deposits are not typical of hydrothermal Mn deposits. In particular at least some of the crusts have developed on a sediment or unconsolidated talus substrate. Some crusts, or specific layers within some crusts, display a chemical composition which suggests a significant input from normal seawater.

Chemistry of interstitial waters of ODP Site 166-718

Preliminary data are presented on dissolved heavy metals in interstitial water samples collected at Site 718 of Ocean Drilling Program Leg 118. The heavy metals at this site are divided into three groups: Group I (B, K, Mn, Ni, Pb, total Si, total P, V) behaves like Mg, which decrease with depth; Group II (Ba, Cu, Sr, Ti) behaves like Ca, which increases with depth; and Group 111 (Cd, Co, Cr, Fe, Na, Mo, Zn) contains metals that are independent of depth. Mg decreases with depth from 50 mM at the seafloor to 21 mM at 900 mbsf. Mn in the sulfate reduction zone (1.0 to 2.8 ppm) is more highly concentrated than in the methane fermentation zone (0.23 to 0.50 ppm), except for Section 116-718-1H-1. A similar behavior is also observed for V and Pb. Ni, B, and K decrease non-uniformly with depth. Ca and Sr increase with depth at the same rates, indicating the dissolution of inorganic calcium carbonate by anaerobic oxidation of organic matter (Sayles, 1981, doi:10.1016/0016-7037(81)90132-0). The distribution of Ba with depth is very similar to those of Ca and Sr. Cu and Ti profiles trend to increase non-uniformly with depth. Fe is constant with depth. The sharp decrease in total silicate concentration at the seafloor probably indicates a decrease in the decomposition of siliceous biological matter (e.g., diatoms) and production of opal. The constant levels of Group 111, except for Na and Fe, may reveal equal sources of supply from surface seawater and the Himalayas over time.

Organic carbon content, stable isotopes and planktonic foraminifera in sediments of tghe norther Arabian Sea oxygen minimum zone

The northern Arabian Sea is one of the few regions in the open ocean where thermocline water is severely depleted in oxygen. The intensity of this oxygen minimum zone (OMZ) has been reconstructed over the past 225,000 years using proxies for surface water productivity, water column denitrification, winter mixing, and the aragonite compensation depth (ACD). Changes in OMZ intensity occurred on orbital and suborbital timescales. Lowest O2 levels correlate with productivity maxima and shallow winter mixing. Precession-related productivity maxima lag early summer insolation maxima by ~6 kyr, which we attribute to a prolonged summer monsoon season related to higher insolation at the end of the summer. Periods with a weakened or even non-existent OMZ are characterized by low productivity conditions and deep winter mixing attributed to strong and cold winter monsoonal winds. The timing of deep winter mixing events corresponds with that of periods of climatic cooling in the North Atlantic region.

Rare earth element contents in deposits and minerals of the ocean floor

Current understanding of rare earth element (REE) geochemistry in the ocean is given in the book. Chemical properties determining REE migration ability in natural processes, sources of REE in the ocean, behavior of REE in river-sea mixing zones, fractionation of dissolved and particulate REE in ocean waters under aerobic and anaerobic conditions, distribution of REE in terrigenous, authigenic, hydrothermal and biogenic sediment components (clay, bone detritus, barite, phillipsite, Fe- and Mn-oxyhydroxides, Fe-Ca hydroxophosphate, diatoms and foraminiferas) are under consideration.

Rare earth element contents and sources in phosphorites from Pacific seamounts

Distribution of rare earth elements (REE) was studied in phosphorites collected from seamounts at depths from about 400 to 3600 m. In general phosphorites are characterized by high REE con¬tent, by a strong negative Ce anomaly, by a slight positive Gd anomaly, and by slight enrichment in heavy REE, which is also characteristic of seawater, where, to certain extent, composition of REE depends on depth. Comparison of REE composition in phosphorites and in seawater from the Northwest Pacific by means of Q-mode factor analysis revealed that REE have been transported into the phosphorites from various water depths following submergence of the seamounts. This corresponds to paleotectonic reconstructions, but is only partially consistent with age determinations of phosphorites.

Halogene concentrations and iodine isotopic composition in pore water on Hydrare Ridge, Cascadia continental margin

We report iodine and bromine concentrations in a total of 256 pore water samples collected from all nine sites of Ocean Drilling Program Leg 204, Hydrate Ridge. In a subset of these samples, we also determined iodine ages in the fluids using the cosmogenic isotope 129I (T1/2 = 15.7 Ma). The presence of this cosmogenic isotope, combined with the strong association of iodine with methane, allows the identification of the organic source material responsible for iodine and methane in gas hydrates. In all cores, iodine concentrations were found to increase strongly with depth from values close to that of seawater (0.0004 mM) to concentrations >0.5 mM. Several of the cores taken from the northwest flank of the southern summit show a pronounced maximum in iodine concentrations at depths between 100 and 150 meters below seafloor in the layer just above the bottom-simulating reflector. This maximum is especially visible at Site 1245, where concentrations reach values as high as 2.3 mM, but maxima are absent in the cores taken from the slope basin sites (Sites 1251 and 1252). Bromine concentrations follow similar trends, but enrichment factors for Br are only 4-8 times that of seawater (i.e., considerably lower than those for iodine). Iodine concentrations are sufficient to allow isotope determinations by accelerator mass spectrometry in individual pore water samples collected onboard (~5 mL). We report 129I/I ratios in a few samples from each core and a more complete profile for one flank site (Site 1245). All 129I/I ratios are below the marine input ratio (Ri = 1500x10**-15). The lowest values found at most sites are between 150 and 250x10**-15, which correspond to minimum ages between 40 and 55 Ma, respectively. These ages rule out derivation of most of the iodine (and, by association, of methane) from the sediments hosting the gas hydrates or from currently subducting sediments. The iodine maximum at Site 1245 is accompanied by an increase in 129I/I ratios, suggesting the presence of an additional source with an age younger than 10 Ma; there is indication that younger sources also contribute at other sites, but data coverage is not yet sufficient to allow a definitive identification of sources there. Likely sources for the older component are formations of early Eocene age close to the backstop in the overriding wedge, whereas the younger sources might be found in recent sediments underlying the current locations of the gas hydrates.