Terrigenous input and marine productivity for the Pacific sector of the Southern Ocean based on lipid biomarkers

In this study, we present a new multiproxy data set of terrigenous input, marine productivity and sea surface temperature (SST) from 52 surface sediment samples collected along E-W transects in the Pacific sector of the Southern Ocean. Allochtonous terrigenous input was characterized by the distribution of plant wax n-alkanes and soil-derived branched glycerol dialkyl glycerol tetraethers (brGDGTs). 230Th-normalized burial rates of both compound groups were highest close to the potential sources in Australia and New Zealand and are strongly related to lithogenic contents, indicating common sources and transport. Detection of both long-chain n-alkanes and brGDGTs at the most remote sites in the open ocean strongly suggests a primarily eolian transport mechanism to at least 110°W, i.e. by prevailing westerly winds. Two independent organic SST proxies were used, the UK'37 based on long-chain alkenones, and the TEX86 based on isoprenoid GDGTs. Both, UK'37 and TEX86 indices show robust relationships with temperature over a temperature range between 0.5 and 20°C, likely implying different seasonal and regional imprints on the temperature signal. While alkenone-based temperature estimates reliably reflect modern SST even at the low temperature end, large temperature residuals are observed for the polar ocean using the TEX86 index. 230Th-normalized burial rates of alkenones are highest close to the Subtropical Front and are positively related to lithogenic fluxes throughout the study area. In contrast, highest isoGDGT burial south of the Antarctic Polar Front is not related with dust flux but may be largely controlled by diatom blooms, and thus high opal fluxes during austral summer.

Description of Mn deposits and chemical composition of altered pelagic sediments of sites surveyed by R/V Vityaz in the Southern Basin of the Pacific Ocean

The object of the detailed investigations was an unusual material collected in the region of the Southern Basin of the Pacific Ocean floor, with features of intense manifestation of volcanic processes and subsequent hydrothermal alterations. These processes to a significant degree transformed the ferromanganese nodules and the pelagic sediments, causing the development of a new type of oceanic manganese mineralization.

Benthic foraminiferal stable isotope record from ODP Site 138-849 on the East Pacific Rise

Benthic foraminifer and delta13C data from Site 849, on the west flank of the East Pacific Rise (0°11 'N, 110°31'W; 3851 m), give relatively continuous records of deep Pacific Ocean stable isotope variations between 0 and 5 Ma. The mean sample spacing is 4 k.y. Most analyses are from Cibicides wuellerstorfi, but isotopic offsets relative to Uvigerina peregrina appear roughly constant. Because of its location west of the East Pacific Rise, Site 849 yields a suitable record of mean Pacific Ocean delta13C, which approximates a global oceanic signal. The ~100-k.y.-period climate cycle, which is prevalent in delta18O does not dominate the long-term delta13C record. For delta13C, variations in the ~400- and 41-k.y. periods are more important. Phase lags of delta13C relative to ice volume in the 41- and 23-k.y. bands are consistent with delta13C as a measure of organic biomass. A model-calculated exponential response time of 1-2 k.y. is appropriate for carbon stored in soils and shallow sediments responding to glacial-interglacial climate change. Oceanic delta13C leads ice volume slightly in the 100-k.y. band, and this suggests another process such as changes in continental weathering to modulate mean river delta13C at long periods. The delta13C record from Site 849 diverges from that of Site 677 in the Panama Basin mostly because of decay of 13C-depleted organic carbon in the relatively isolated Panama Basin. North Atlantic to Pacific delta13C differences calculated using published data from Sites 607 and 849 reveal variations in Pliocene deep water within the range of those of the late Quaternary. Maximum delta13C contrast between these sites, which presumably reflects maximum influx of high-delta13C northern source water into the deep North Atlantic Ocean, occurred between 1.3 and 2.1 Ma, well after the initiation of Northern Hemisphere glaciation. Export of high-delta13C North Atlantic Deep Water from the Atlantic to the circumpolar Antarctic, as recorded by published delta13C data from Subantarctic Site 704, appears unrelated to the North Atlantic-Pacific delta13C contrast. To account for this observation, we suggest that deep-water formation in the North Atlantic reflects northern source characteristics, whereas export of this water into the circumpolar Antarctic reflects Southern Hemisphere wind forcing. Neither process appears directly linked to ice-volume variations.

Neogene calcareous nannofossils in the Caribbean and the eastern equatorial Pacific

Principal components analysis of nannofossil assemblages in five ODP sites in the Caribbean and the eastern equatorial Pacific was used to monitor Neogene surface water circulation changes. In 15.83-10.71 Ma, floral distributions clearly show the existence of the Circum-tropical Current between the Caribbean and the eastern equatorial Pacific and no surface water communication between the northern and southern Caribbean. In 10.71-9.36 Ma, the Circum-tropical Current weakened and the northward intra-Caribbean current had been initiated. Northern and southern Caribbean surface waters again became separated in 8.35-3.65 Ma and the Circum-tropical Current was regenerated. After 2.76 Ma, the northward intra-Caribbean current has completely been established and the Circum-tropical Current disappeared. This suggests that a barrier to the Circum-tropical Current had formed, indicating the final closure of the Isthmus of Panama.

Age models of sediment cores from the Southern Ocean

Recent geochemical models invoke ocean alkalinity changes, particularly in the surface Southern Ocean, to explain glacial age pCO2 reduction. In such models, alkalinity increases in glacial periods are driven by reductions in North Atlantic Deep Water (NADW) supply, which lead to increases in deep-water nutrients and dissolution of carbonate sediments, and to increased alkalinity of Circumpolar Deep Water upwelling in the surface Southern Ocean. We use cores from the Southeast Indian Ridge and from the deep Cape Basin in the South Atlantic to show that carbonate dissolution was enhanced during glacial stages in areas now bathed by Circumpolar Deep Water. This suggests that deep Southern Ocean carbonate ion concentrations were lower in glacial stages than in interglacials, rather than higher as suggested by the polar alkalinity model [Broecker and Peng, 1989, doi:10.1029/GB001i001p00015]. Our observations show that changes in Southern Ocean CaCO3 preservation are coherent with changes in the relative flux of NADW, suggesting that Southern Ocean carbonate chemistry is closely linked to changes in deepwater circulation. The pattern of enhanced dissolution in glacials is consistent with a reduction in the supply of nutrient-depleted water (NADW) to the Southern Ocean and with an increase of nutrients in deep water masses. Carbonate mass accumulation rates on the Southeast Indian Ridge (3200-3800 m), and in relatively shallow cores (<3000 m) from the Kerguelen Plateau and the South Pacific were significantly reduced during glacial stages, by about 50%. The reduced carbonate mass accumulation rates and enhanced dissolution during glacials may be partly due to decreases in CaCO3:Corg flux ratios, acting as another mechanism which would raise the alkalinity of Southern Ocean surface waters. The polar alkalinity model assumes that the ratio of organic carbon to carbonate production on surface alkalinity is constant. Even if overall productivity in the Southern Ocean were held constant, a decrease in the CaCO3:Corg ratio would result in increased alkalinity and reduced pCO2 in Southern Ocean surface waters during glacials. This ecologically driven surface alkalinity change may enhance deepwater-mediated changes in alkalinity, and amplify rapid changes in pCO2.

Exchange capacity and chemical composition of Fe-Mn nodules from the Central Pacific

Constants of calcium, magnesium, zinc, cobalt, copper, and nickel exchange for sodium in iron-manganese nodules taken from different parts of the Pacific Ocean were determined under static conditions at constant ionic strength (?=0.05). These determinations revealed high capacity of nodules for sorbing the referred ions (their exchange constants range from 1.93 to 20.85). Obtained data demonstrate the major role of MnO and Fe2O3 in sorption processes in iron-manganese nodules.

Nd, Pb, and Be isotopes of ferromanaganese crusts of the South Pacific

The application of radiogenic isotopes to the study of Cenozoic circulation patterns in the South Pacific Ocean has been hampered by the fact that records from only equatorial Pacific deep water have been available. We present new Pb and Nd isotope time series for two ferromanganese crusts that grew from equatorial Pacific bottom water (D137-01, 'Nova', 7219 m water depth) and southwest Pacific deep water (63KD, 'Tasman', 1700 m water depth). The crusts were dated using 10Be/9Be ratios combined with constant Co-flux dating and yield time series for the past 38 and 23 Myr, respectively. The surface Nd and Pb isotope distributions are consistent with the present-day circulation pattern, and therefore the new records are considered suitable to reconstruct Eocene through Miocene paleoceanography for the South Pacific. The isotope time series of crusts Nova and Tasman suggest that equatorial Pacific deep water and waters from the Southern Ocean supplied the dissolved trace metals to both sites over the past 38 Myr. Changes in the isotopic composition of crust Nova are interpreted to reflect development of the Antarctic Circumpolar Current and changes in Pacific deep water circulation caused by the build up of the East Antarctic Ice Sheet. The Nd isotopic composition of the shallower water site in the southwest Pacific appears to have been more sensitive to circulation changes resulting from closure of the Indonesian seaway.

Plio-Pleistocene age-depth points and occurrence of hiatuses in ODP sites of the Southern Ocean

Changes in Atlantic deep water circulation were reconstructed by comparing the benthic foraminiferal delta13C record at ODP Site 1090 in the South Atlantic with similar records from the North Atlantic (Sites 982, 607, 925, 929) and deep Pacific (Site 849) oceans. Important deep water circulation changes occurred in the early Pleistocene at 1.55 Myr and during the Mid-Pleistocene Transition at 0.9 Myr. At 1.55 Myr, glacial delta13C values in the Southern Ocean became significantly lower than those in the deep Pacific, establishing a pattern that persisted throughout the late Pleistocene. We propose that the lowering of delta13C values of Southern Component Water (SCW) at this time resulted from expansion of sea ice and reduced ventilation of deep water during glacial periods after marine isotope stage 52. Accompanying this change in Southern Ocean deep water circulation was enhanced interhemispheric coupling between the North and South Atlantic after 1.55 Myr. At ~0.9 Myr, the magnitude of glacial-to-interglacial variabilityin delta13C increased and shifted to a longer frequency (100 kyr) along with oceanic delta18O (ice volume). Calculation of percent Northern Component Water (NCW) using Site 1090 as the SCW end member yielded 20-30% less reduction of NCW during glacial periods of the late Pleistocene. Also, a trend toward reduced glacial suppression of NCW during the past 400 kyr is not evident. The apparent decoupling of ice volume and deep water circulation reported previously maybe an artifact of using a Pacific, rather than a Southern Ocean, carbon isotopic record to calculate past mixing ratios of NCW and SCW.

Barium concentration in sediments of the central Equatorial Pacific

High resolution pore-water dissolved Ba concentration-depth profiles were determined at seven sites across an Equatorial Pacific productivity gradient from 12°S to 9°N, at 140°W. These data are important for understanding the physical, chemical, and biological controls on Ba recycling in the ocean, and for evaluating the paleo-oceanographic significance of Ba content in central Equatorial Pacific sediments. Pore-water Ba concentrations at all sites are higher than in the overlying bottom water, leading to a diffusive flux of Ba into the ocean. A pronounced subsurface concentration maximum exceeding barite solubility characterizes the dissolved Ba pore-water profiles, suggesting that the Ba regenerated in the upper few millimeters of sediment is not controlled by barite solubility. A few centimeters down-core Ba concentrations reach a relatively constant value of approximately barite saturation. The benthic Ba flux shows a clear zonal trend, with a maximum between 2°S and 2°N, most probably due to higher productivity at the equatorial divergence zone, and with lowest values at the southern and northern extremes of the transect. The dissolved Ba flux between 2°S and 2°N is ~30 nmol/cm**2 yr and drops to 6 nmol/cm**2 yr at 12°S. Even the lowest fluxes are significantly higher than those previously reported for the open ocean. In the Equatorial Pacific the calculated Ba recycling efficiency is about 70%. Thus, ~30% of the particulate Ba flux to the deep ocean is preserved in the sediments, compared with less than 1% for organic carbon and ~5% for biogenic silica. Mass balance calculation of the oceanic Ba cycle, using a two-box model, implies benthic Ba fluxes similar to those reported here for a steady-state ocean.