Chemical and isotopic compositions of altered MORB from ODP Holes 187-1154A, 187-1155B, and 187-1160B

Boron and Pb isotopic compositions together with B-U-Th-Pb concentrations were determined for Pacific and Indian mantle-type mid-ocean ridge basalts (MORB) obtained from shallow drill holes near the Australian Antarctic Discordance (AAD). Boron contents in the altered samples range from 29.7 to 69.6 ppm and are extremely enriched relative to fresh MORB glass with 0.4-0.6 ppm B. Similarly the d11B values range from 5.5? to 15.9? in the altered basalts and require interaction with a d11B enriched fluid similar to seawater ~39.5? and/or boron isotope fractionation during the formation of secondary clays. Positive correlations between B concentrations and other chemical indices of alteration such as H2O CO2, K2O, P2O5, U and 87Sr/86Sr indicate that B is progressively enriched in the basalts as they become more altered. Interestingly, d11B shows the largest isotopic shift to +16? in the least altered basalts, followed by a continual decrease to +5-6? in the most altered basalts. These observations may indicate a change from an early seawater dominated fluid towards a sediment-dominated fluid as a result of an increase in sediment cover with increasing age of the seafloor. The progression from heavy d11B towards lighter values with increasing degrees of alteration may also reflect increased formation of clay minerals (e.g., saponite). A comparison of 238U/204Pb and 206Pb/204Pb in fresh glass and variably altered basalt from Site 1160B shows extreme variations that are caused by secondary U enrichment during low temperature alteration. Modeling of the U-Pb isotope system confirms that some alteration events occurred early in the 21.5 Ma history of these rocks, even though a significant second pulse of alteration happened at ~12 Ma after formation of the crust. The U-Pb systematics of co-genetic basaltic glass and variably low temperature altered basaltic whole rocks are thus a potential tool to place age constraints on the timing of alteration and fluid flow in the ocean crust.

Ages analysis, trace elements and relative frequencies of foraminifera snd diatoms from sediment cores AvH248260-2 and AvH248260-1, Ameralik Fjord, SW Greenland

A multiproxy record including benthic foraminifera, diatoms and XRF data of a marine sediment core from a SW Greenland fjord provides a detailed reconstruction of the oceanographic and climatic variations of the region during the last 4400 cal. years. The lower part of our record represents the final termination of the Holocene Thermal Maximum. After the onset of the 'Neoglaciation' at approximately 3.2 ka cal. BP, the fjord system was subject to a number of marked hydrographical changes that were closely linked to the general climatic and oceanographic development of the Labrador Sea and the North Atlantic region. Our data show that increased advection of Atlantic water (Irminger Sea Water) from the West Greenland Current into the Labrador Sea was a typical feature of Northeast Atlantic cooling episodes such as the 'Little Ice Age' and the 'European Dark Ages', while the advection of Irminger Sea Water decreased significantly during warm episodes such as the 'Mediaeval Warm Period' and the 'Roman Warm Period'.Whereas the 'Mediaeval Warm Period' was characterized by relatively cool climate as suggested by low meltwater production, the preceding 'Dark Ages' display higher meltwater runoff and consequently warmer climate. When compared with European climate, these regional climate anomalies indicate persisting patterns of advection of colder, respectively warmer air masses in the study region during these periods and thus a long-term seesaw climate pattern between West Greenland and Europe.

Holocene flood frequency across the Central Alps

The frequency of large-scale heavy precipitation events in the European Alps is expected to undergo substantial changes with current climate change. Hence, knowledge about the past natural variability of floods caused by heavy precipitation constitutes important input for climate projections. We present a comprehensive Holocene (10,000 years) reconstruction of the flood frequency in the Central European Alps combining 15 lacustrine sediment records. These records provide an extensive catalog of flood deposits, which were generated by flood-induced underflows delivering terrestrial material to the lake floors. The multi-archive approach allows suppressing local weather patterns, such as thunderstorms, from the obtained climate signal. We reconstructed mainly late spring to fall events since ice cover and precipitation in form of snow in winter at high-altitude study sites do inhibit the generation of flood layers. We found that flood frequency was higher during cool periods, coinciding with lows in solar activity. In addition, flood occurrence shows periodicities that are also observed in reconstructions of solar activity from 14C and 10Be records (2500-3000, 900-1200, as well as of about 710, 500, 350, 208 (Suess cycle), 150, 104 and 87 (Gleissberg cycle) years). As atmospheric mechanism, we propose an expansion/shrinking of the Hadley cell with increasing/decreasing air temperature, causing dry/wet conditions in Central Europe during phases of high/low solar activity. Furthermore, differences between the flood patterns from the Northern Alps and the Southern Alps indicate changes in North Atlantic circulation. Enhanced flood occurrence in the South compared to the North suggests a pronounced southward position of the Westerlies and/or blocking over the northern North Atlantic, hence resembling a negative NAO state (most distinct from 4.2 to 2.4 kyr BP and during the Little Ice Age). South-Alpine flood activity therefore provides a qualitative record of variations in a paleo-NAO pattern during the Holocene. Additionally, increased South Alpine flood activity contrasts to low precipitation in tropical Central America (Cariaco Basin) on the Holocene and centennial time scale. This observation is consistent with a Holocene southward migration of the Atlantic circulation system, and hence of the ITCZ, driven by decreasing summer insolation in the Northern hemisphere, as well as with shorter-term fluctuations probably driven by solar activity.

Radiocarbon ages, dose rates, water isotopic composition and hydrochemistry of samples from Komakuk Beach and Herschel Island

Terrestrial permafrost archives along the Yukon Coastal Plain (northwest Canada) have recorded landscape development and environmental change since the Late Wisconsinan at the interface of unglaciated Beringia (i.e. Komakuk Beach) and the northwestern limit of the Laurentide Ice Sheet (i.e. Herschel Island). The objective of this paper is to compare the late glacial and Holocene landscape development on both sides of the former ice margin based on permafrost sequences and ground ice. Analyses at these sites involved a multi-proxy approach including: sedimentology, cryostratigraphy, palaeoecology of ostracods, stable water isotopes in ground ice, hydrochemistry, and AMS radiocarbon and infrared stimulated luminescence (IRSL) dating. AMS and IRSL age determinations yielded full glacial ages at Komakuk Beach that is the northeastern limit of ice-free Beringia. Herschel Island to the east marks the Late Wisconsinan limit of the northwest Laurentide Ice Sheet and is composed of ice-thrust sediments containing plant detritus as young as 16.2 cal ka BP that might provide a maximum age on ice arrival. Late Wisconsinan ice wedges with sediment-rich fillings on Herschel Island are depleted in heavy oxygen isotopes (mean d18O of -29.1 per mil); this, together with low d-excess values, indicates colder-than-modern winter temperatures and probably reduced snow depths. Grain-size distribution and fossil ostracod assemblages indicate that deglaciation of the Herschel Island ice-thrust moraine was accompanied by alluvial, proluvial, and eolian sedimentation on the adjacent unglaciated Yukon Coastal Plain until ~11 cal ka BP during a period of low glacio-eustatic sea level. The late glacial-Holocene transition was marked by higher-than-modern summer temperatures leading to permafrost degradation that began no later than 11.2 cal ka BP and caused a regional thaw unconformity. Cryostructures and ice wedges were truncated while organic matter was incorporated and soluble ions were leached in the thaw zone. Thermokarst activity led to the formation of ice-wedge casts and deposition of thermokarst lake sediments. These were subsequently covered by rapidly accumulating peat during the early Holocene Thermal Maximum. A rising permafrost table, reduced peat accumulation, and extensive ice-wedge growth resulted from climate cooling starting in the middle Holocene until the late 20th century. The reconstruction of palaeolandscape dynamics on the Yukon Coastal Plain and the eastern Beringian edge contributes to unraveling the linkages between ice sheet, ocean, and permafrost that have existed since the Late Wisconsinan.

SHRIMP Pb/U zircon ages of plutonic rocks from Southwest Indian Ridge

Absolute ages of plutonic rocks from mid-ocean ridges provide important constraints on the scale, timing and rates of oceanic crustal accretion, yet few such rocks have been absolutely dated. We present 206Pb/238U SHRIMP zircon ages from two ODP Drill Holes and a surface sample from Atlantis Bank on the Southwest Indian Ridge. We report ten new sample ages from 26-1430 m in ODP Hole 735B, and one from 57 m in ODP Hole 1105A. Including a previously published age, eleven samples from Hole 735B yield 206Pb/238U zircon crystallization ages that are the same, within error, overlap with the estimated magnetic age and are inferred to date the main period of crustal growth, the average age of analyses is 11.99 ± 0.12 Ma. Any differences in the ages of magmatic series and/or tectonic blocks within Hole 735B are unresolvable and eight well-constrained ages vary from 11.86 ± 0.20 Ma to 12.13 ± 0.21 Ma, a range of 0.27 ± 0.29 Ma, consistent with the duration of crustal accretion observed at the Mid-Atlantic Ridge. An age of 11.87 ± 0.23 Ma from Hole 1105A is within error of ages from Hole 735B and permits previous correlations made between zones of oxide-rich gabbros in each hole. Pb/U zircon ages > 0.5 Ma younger than the magnetic age are recorded in at least three samples from Atlantis Bank, one from Hole 735B and two collected along a fault scarp to the East. These young ages may date one or more off-axis events previously suggested from thermochronologic data and support the interpretation of a complex geological history following crustal accretion at Atlantis Bank. Together with results from the surface of Atlantis Bank, dating has shown that while the majority of Pb/U SHRIMP zircon ages record the short-lived (< 0.5 Ma) phase of crustal accretion on-axis, results from several samples precede and post-date this period by > 1 Ma suggesting a complex and prolonged magmatic/tectonic history for the crust at Atlantis Bank.

Sea-surface temperature reconstruction for sediment core GIK18287-3 in the tropical South China Sea

The timing and magnitude of sea-surface temperature (SST) changes in the tropical southern South China Sea (SCS) during the last 16,500 years have been reconstructed on a high-resolution, 14C-dated sediment core using three different foraminiferal transfer functions (SIMMAX28, RAM, FP-12E) and geochemical (Uk'37) SST estimates. In agreement with CLIMAP reconstructions, both the FP-12E and the Uk'37 SST estimates show an average late glacial-interglacial SST difference of 2.0°C, whereas the RAM and SIMMAX28 foraminiferal transfer functions show only a minor (0.6°C) or no consistent late glacial-interglacial SST change, respectively. Both the Uk'37 and the FP-12E SST estimates, as well as the planktonic foraminiferal delta18O values, indicate an abrupt warming (ca. 1°C in <200 yr) at the end of the last glaciation, synchronous (within dating uncertainties) with the Bølling transition as recorded in the Greenland Ice Sheet Project 2 (GISP2) ice core, whereas the RAM-derived deglacial SST increase appears to lag during this event by ca. 500 yr. The similarity in abruptness and timing of the warming associated with the Bølling transition in Greenland and the southern SCS suggest a true synchrony of the Northern Hemisphere warming at the end of the last glaciation. In contrast to the foraminiferal transfer function estimates that do not indicate any consistent cooling associated with the Younger Dryas (YD) climate event in the tropical SCS, the Uk'37 SST estimates show a cooling of ca. 0.2-0.6°C compared to the Bølling-Allerød period. These Uk'37 SST estimates from the southern SCS argue in favor of a Northern Hemisphere-wide, synchronous cooling during the YD period.

Age determination and stable isotope records of sediment core M78/1_235-1

Paleoenvironmental studies and climate models demonstrate that fluvial runoff and moisture availability in the Caribbean hinterland react very sensitively to climatic variations. Late Pleistocene and Holocene climate records document pronounced dry and wet periods over tropical South America mainly caused by shifts of the Intertropical Convergence Zone (ITCZ). However, forcing mechanisms for changes in the ITCZ position remain controversial. Here we present high-resolution foraminiferal Ba/Ca and d18Oseawater records from a core located within the Orinoco River outflow documenting abrupt hydrological changes in the Orinoco catchment area during the deglacial and Holocene. Our data, obtained from the surface-dwelling foraminifera Globigerinoides ruber (pink), show an abrupt increase in Ba/Ca ratios in the early Holocene, starting ~600 yr after the end of the Younger Dryas (YD) cold interval at ca. 10.8 ka and suggesting a massive reorganization of moisture sources in northern South America. In contrast, the salinity dependent d18Oseawater from the same samples shows a gradual decrease starting at the end of the YD. The offset of our Ba/Ca peak excludes meltwater release in conjunction with the northern Andean glacier retreat well before the end of the YD as a forcing mechanism. We suggest that the Ba/Ca record documents an abrupt increase in Ba-rich waters of a northern Andean source caused by the insolation-driven shift of the ITCZ and/or enhanced monsoon activity.

Isotopic geochemistry of lavas at DSDP Holes 81-553 and 81-555

It is demonstrated by K-Ar analyses that the age of reversely magnetized basalts, which immediately predate magnetic Anomaly 24B, is 53.5 ± 1.9 m.y. Samples from deep levels appear to be grossly contaminated by an extraneous argon component with a uniform argon-40/argon-36 ratio 440. This component is thought to have been derived from fluids circulating in the lava pile during burial. The age result corroborates the assignment previously made to Anomaly 24B by Hailwood et al. (1979) and Lowrie and Alvarez (1981). It additionally suggests that lava extrusion formed part of a much larger magmatic event, which affected wide areas of the North Atlantic margins around the Paleocene/Eocene boundary, and can therefore probably be considered a good estimate of the age of this boundary. Initial 143Nd/144Nd ratios lie in the very restricted range 0.512920 ± 19 to 0.513026 ± 24 and initial 8 7Sr/86Sr ratios from ca. 0.703 to ca. 0.705. Acid leaching reduces the latter range to 0.70264 ± 4 to 0.70384 ± 4, suggesting that the higher 87Sr/86Sr ratios resulted from interaction with seawater. The array of data for treated samples is closely conformable on a 143Nd/144Nd-87Sr/86Sr diagram with the main oceanic mantle array and with previously published fields for Atlantic Ocean basalts. No evidence for any continental crustal contamination has been found. This suggests, but does not prove, that continental crust played no part in the genesis of these rocks.

10Be/9Be-based chronostratigraphy of ODP Hole 181-1121B from the Southwest Pacific Ocean

A 10Be/9Be-based chronostratigraphy has been determined for ODP 181, Site 1121 sediment core, recovered from the foot of the Campbell Plateau, Southwest Pacific Ocean. This core was drilled through the Campbell 'skin drift' in ca. 4500 m water depth on the mid-western margin of the extensive Campbell Nodule Field, beneath the flow of the major cold-water Deep Western Boundary Current (DWBC). In the absence of detailed biostratigraphy, beryllium isotopes have provided essential time information to allow palaeo-environmental interpretation to be undertaken on the upper 7 m of the core. Measured 10Be/9Be ratios of sediment, and of ferromanganese nodules entrapped in the sediment, decrease systematically with depth in the core, in accordance with radioactive decay. However, the 10Be/9Be data diverge from ca. 3 m below the seafloor (mbsf) to the top of the core, giving rise to several possible geochronological models. The preferred model assumes that the measured 10Be/9Be ratios of the nodule rims reflect initial 10Be/9Be ratios equivalent to contemporary seawater, and that these can be used to derive the true age of the sediment where the nodules occur. The nodule rim ages can be then used to interpret the sediment 10Be/9Be data, which indicate an overall age to ca. 7 mbsf of ca. 17.5 Ma. The derived chronology is consistent with diatom biostratigraphy, which indicates an age of 2.2-3.6 Ma at 1 mbsf. Calculated sedimentation rates range from 8 to 95 cm m.y.**-1, with an overall rate to 7 mbsf of ca. 39 cm m.y.**-1. The lowest rates generally coincide with the occurrence of entrapped nodules, and reflect periods of increased bottom current flow causing net sediment loss. Growth rates of individual nodules decrease towards the top of the sediment core, similar to the observed decrease in growth rate from core to rim of seafloor nodules from the Campbell Nodule Field. This may be related to an overall increase in the vigour of the DWBC from ca. 10 Ma to the present.

Extraterrestrial 3He estimation across the Paleocene-Eocene thermal maximum at ODP Site 208-1266

In the deep-sea, the Paleocene-Eocene Thermal Maximum (PETM) is often marked by clay-rich condensed intervals caused by dissolution of carbonate sediments, capped by a carbonate-rich interval. Constraining the duration of both the dissolution and subsequent cap-carbonate intervals is essential to computing marine carbon fluxes and thus testing hypotheses for the origin of this event. To this end, we provide new high-resolution helium isotope records spanning the Paleocene-Eocene boundary at ODP Site 1266 in the South Atlantic. The extraterrestrial 3He, 3HeET, concentrations replicate trends observed at ODP Site 690 by Farley and Eltgroth (2003, doi:10.1016/S0012-821X(03)00017-7). By assuming a constant flux of 3HeET we constrain relative changes in accumulation rates of sediment across the PETM and construct a new age model for the event. In this new chronology the zero carbonate layer represents 35 kyr, some of which reflects clay produced by dissolution of Paleocene (pre-PETM) sediments. Above this layer, carbonate concentrations increase for ~165 kyr and remain higher than in the latest Paleocene until 234 +48/-34 kyr above the base of the clay. The new chronology indicates that minimum d13C values persisted for a maximum of 134 +27/-19 kyr and the inflection point previously chosen to designate the end of the CIE recovery occurs at 217 +44/-31 kyr. This allocation of time differs from that of the cycle-based age model of Röhl et al. (2007, doi:10.1029/2007GC001784) in that it assigns more time to the clay layer followed by a more gradual recovery of carbonate-rich sedimentation. The new model also suggests a longer sustained d13C excursion followed by a more rapid recovery to pre-PETM d13C values. These differences have important implications for constraining the source(s) of carbon and mechanisms for its subsequent sequestration, favoring models that include a sustained release

Stable hydrogen and carbon isotope records and concentration data for four long chain n-alkanes from core GIK16160-3 derived from the Zambezi delta in the Mozambique Channel during the last 37,000 years

Hydrogen isotope values (dD) of sedimentary terrestrial leaf wax such as n-alkanes or n-acids have been used to map and understand past changes in rainfall amount in the tropics because dD of precipitation is commonly assumed as the first order controlling factor of leaf wax dD. Plant functional types and their photosynthetic pathways can also affect leaf wax dD but these biological effects are rarely taken into account in paleo studies relying on this rainfall proxy. To investigate how biological effects may influence dD values we here present a 37,000-year old record of dD and stable carbon isotopes (d13C) measured on four n-alkanes (n-C27, n-C29, n-C31, n-C33) from a marine sediment core collected off the Zambezi River mouth. Our paleo d13C records suggest that each individual n-alkanes had different C3/C4 proportional contributions. n-C29 was mostly derived from a C3 dicots (trees, shrubs and forbs) dominant vegetation throughout the entire record. In contrast, the longer chain n-C33 and n-C31 were mostly contributed by C4 grasses during the Glacial period but shifted to a mixture of C4 grasses and C3 dicots during the Holocene. Strong correlations between dD and d13C values of n-C33 (correlation coefficient R2 = 0.75, n = 58) and n-C31 (R2 = 0.48, n = 58) suggest that their dD values were strongly influenced by changes in the relative contributions of C3/C4 plant types in contrast to n-C29 (R2 = 0.07, n = 58). Within regions with variable C3/C4 input, we conclude that dD values of n-C29 are the most reliable and unbiased indicator for past changes in rainfall, and that dD and d13C values of n-C31 and n-C33 are sensitive to C3/C4 vegetation changes. Our results demonstrate that a robust interpretation of palaeohydrological data using n-alkane dD requires additional knowledge of regional vegetation changes from which nalkanes are synthesized, and that the combination of dD and d13C values of multiple n-alkanes can help to differentiate biological effects from those related to the hydrological cycle.

Age model, radiocarbon dates and dinoflagellate cyst measurements from sediment core DA06-139G

A marine sediment core from Vaigat in Disko Bugt, West Greenland, has been analysed in terms of lithology, dinoflagellate cysts and foraminifera in order to evaluate the influence of oceanographic variability on West Greenland glacier stability. The data show that during the past 5200 years the Atlantic foraminiferal abundance in the subsurface waters of the West Greenland Current (WGC) episodically increased, indicating periods of increases in the inflow of subsurface warm Atlantic water at 2000 - 1500 cal. yr BP and 1300 cal. yr BP as well as periods of less pronounced increased bottom-water temperatures around 4700 - 4000 cal. yr BP, 3100 - 2800, 2600, 1000 - 800, 500 - 400, and at 200 cal. yr. The sedimentological and dinoflagellate cyst data indicate that these episodes with enhanced advection of Irminger Sea-derived waters are accompanied by increased iceberg rafting, which we link to increased iceberg calving in relation to destabilization of the Jakobshavn Isbrae. The long-term trend in the data documents the end of a late-Holocene Thermal Maximum between 5200 and 4300 cal. yr BP and a final onset of the Neoglaciation at 3500 cal. yr BP. Increased responses of the iceberg rafting after 3500 cal. yr BP, reflects a westward/seaward advance of the glacier margin in relation to onset of Neoglaciation and a development of the glacier into a floating tongue after 2000 cal. yr BP. A comparison of our record with a record from the eastern North Atlantic indicates that a NAO-like anomaly pattern between subsurface waters in West Greenland and atmospheric temperature in the Eastern North Atlantic may have been operating during most of the late Holocene. However, during the past 1000 years the NAO signal may have weakened as some other mode of climate variability overprints the anti-phase climate signal in this region.

Iron isotope and chemical compositions of sediments and basalts drilled seaward of the Mariana Trench

The purpose of this work is to study the mobility and budget of Fe isotopes in the oceanic crust and in particular during low-temperature interaction of seawater with oceanic basalt. We carried out this investigation using samples from Ocean Drilling Program (ODP) Site 801C drilled during Leg 129 and Leg 185 in Jurassic Pacific oceanic crust seaward of the Mariana Trench. The site comprises approximately 450 m of sediment overlying a section of 500 m of basalt, which includes intercalated pelagic and chemical sediments in the upper basaltic units and two low-temperature (10-30°C) ocherous Si-Fe hydrothermal deposits. Fe was chemically separated from 70 selected samples, and 57Fe/54Fe ratios were measured by MC-ICP-MS Isoprobe. The isotopic ratios were measured relative to an internal standard solution and are reported relative to the international Fe-standard IRMM-14. Based on duplicate measurements of natural samples, an external precision of 0.2? (2 sigma) has been obtained. The results indicate that the deep-sea sediment section has a restricted range of d57Fe, which is close to the igneous rock value. In contrast, large variations are observed in the basaltic section with positive d57Fe values (up to 2.05?) for highly altered basalts and negative values (down to ?2.49?) for the associated alteration products and hydrothermal deposits. Secondary Fe-minerals, such as Fe-oxyhydroxides or Fe-bearing clays (celadonite and saponite), have highly variable d57Fe values that have been interpreted as resulting from the partial oxidation of Fe(2+) leached during basalt alteration and precipitated as Fe(3+)-rich minerals. In contrast, altered basalts at Site 801C, which are depleted in Fe (up to 80%), display an increase in d57Fe values relative to fresh values, which suggest a preferential leaching of light iron during alteration. The apparent fractionation factor between dissolved Fe(2+) and Fe remaining in the mineral is from 0.5? to 1.3? and may be consistent with a kinetic isotope fractionation where light Fe is stripped from the minerals. Alternatively, the formation of secondary clays minerals, such as celadonite during basalt alteration may incorporate preferentially the heavy Fe isotopes, resulting in the loss of light Fe isotopes in the fluids. Because microbial processes within the oceanic crust are of potential importance in controlling rates of chemical reactions, Fe redox state and Fe-isotope fractionation, we evaluated the possible effect of this deep biosphere on Fe-isotope signatures. The Fe-isotope systematics presented in this study suggest that, even though iron behavior during seafloor weathering may be mediated by microbes, such as iron-oxidizers, d57Fe variations of more than 4? may also be explained by abiotic processes. Further laboratory experiments are now required to distinguish between various processes of Fe-isotope fractionation during seafloor weathering.

(Table 2) Radiocarbon contents of TOC and alkenone samples

Radiocarbon age relationships between co-occurring planktic foraminifera, alkenones, and total organic carbon in sediments from the continental margins of southern Chile, northwest Africa, and the South China Sea were compared with published results from the Namibian margin. Age relationships between the sediment components are site-specific and relatively constant over time. Similar to the Namibian slope, where alkenones have been reported to be 1000-4500 years older than co-occurring foraminifera, alkenones were significantly (~1000 years) older than co-occurring foraminifera in the Chilean margin sediments. In contrast, alkenones and foraminifera were of similar age (within 2 sigma error or better) in the NW African and South China Sea sediments. Total organic matter and alkenone ages were similar off Namibia (age difference TOC alkenones: 200-700 years), Chile (100-450 years), and NW Africa (360-770 years), suggesting minor contributions of preaged terrigenous material. In the South China Sea, total organic carbon is significantly (2000-3000 years) older owing to greater inputs of preaged terrigenous material. Age offsets between alkenones and planktic foraminifera are attributed to lateral advection of organic matter. Physical characteristics of the depositional setting, such as seafloor morphology, shelf width, and sediment composition, may control the age of co-occurring sediment components. In particular, offsets between alkenones and foraminifera appear to be greatest in deposition centers in morphologic depressions. Aging of organic matter is promoted by transport. Age offsets are correlated with organic richness, suggesting that formation of organic aggregates is a key process.

Hafnium and neodymium in surface waters of the Atlantic sector of the Southern Ocean

Radiogenic isotopes of hafnium (Hf) and neodymium (Nd) are powerful tracers for water mass transport and trace metal cycling in the present and past oceans. However, due to the scarcity of available data the processes governing their distribution are not well understood. Here we present the first combined dissolved Hf and Nd isotope and concentration data from surface waters of the Atlantic sector of the Southern Ocean. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during RV Polarstern expeditions ANT-XXIV/3 and ANT-XXIII/3 in the frame of the International Polar Year (IPY) and the GEOTRACES program. The general distribution of Hf and Nd concentrations in the region is similar. However, at the northernmost station located 200 km southwest of Cape Town a pronounced increase of the Nd concentration is observed, whereas the Hf concentration is minimal, suggesting much less Hf than Nd is released by the weathering of the South African Archean cratonic rocks. From the southern part of the Subtropical Front (STF) to the Polar Front (PF) Hf and Nd show the lowest concentrations (<0.12 pmol/kg and 10 pmol/kg, respectively), most probably due to the low terrigenous flux in this area and efficient scavenging of Hf and Nd by biogenic opal. In the vicinity of landmasses the dissolved Hf and Nd isotope compositions are clearly labelled by terrigenous inputs. Near South Africa Nd isotope values as low as epsilon-Nd = -18.9 indicate unradiogenic inputs supplied via the Agulhas Current. Further south the isotopic data show significant increases to epsilon-Hf = 6.1 and epsilon-Nd = -4.0 documenting exchange of seawater Nd and Hf with the Antarctic Peninsula. In the open Southern Ocean the Nd isotope compositions are relatively homogeneous (epsilon-Nd ~ -8 to -8.5) towards the STF, within the Antarctic Circumpolar Current, in the Weddell Gyre, and the Drake Pasage. The Hf isotope compositions in the entire study area only show a small range between epsilon-Hf = +6.1 and +2.8 support Hf to be more readily released from young mafic rocks compared to old continental ones. The Nd isotope composition ranges from epsilon-Nd = -18.9 to -4.0 showing Nd isotopes to be a sensitive tracer for the provenance of weathering inputs into surface waters of the Southern Ocean.