Chemical composition of interstitial solutions from sediments of DSDP Legs 6-8, 11, 12, 14, 15, and 22

Through the Deep Sea Drilling Project samples of interstitial solutions of deeply buried marine sediments throughout the World Ocean have been obtained and analyzed. The studies have shown that in all but the most slowly deposited sediments pore fluids exhibit changes in composition upon burial. These changes can be grouped into a few consistent patterns that facilitate identification of the diagenetic reactions occurring in the sediments. Pelagic clays and slowly deposited (<1 cm/1000 yr) biogenic sediments are the only types that exhibit little evidence of reaction in the pore waters. In most biogenic sediments sea water undergoes considerable alteration. In sediments deposited at rates up to a few cm/1000 yr the changes chiefly involve gains of Ca(2+) and Sr(2+) and losses of Mg(2+) which balance the Ca(2+) enrichment. The Ca-Mg substitution may often reach 30 mM/kg while Sr(2+) may be enriched 15-fold over sea water. These changes reflect recrystallization of biogenic calcite and the substitution of Mg(2+) for Ca(2+) during this reaction. The Ca-Mg-carbonate formed is most likely a dolomitic phase. A related but more complex pattern is found in carbonate sediments deposited at somewhat greater rates. Ca(2+) and Sr(2+) enrichment is again characteristic, but Mg(2+) losses exceed Ca(2+) gains with the excess being balanced by SO4(post staggered 2-) losses. The data indicate that the reactions are similar to those noted above, except that the Ca(2+) released is not kept in solution but is precipitated by the HCO3(post staggered -) produced in SO4(post staggered 2-) reduction. In both these types of pore waters Na(+) is usually conservative, but K(+) depletions are frequent. In several partly consolidated sediment sections approaching igneous basement contact, very marked interstitial calcium enrichment has been found (to 5.5 g/kg). These phenomena are marked by pronounced depletion in Na(+), Si and CO2, and slight enhancement in Cl(-). The changes are attributed to exchange of Na(+) for Ca(2+) in silicate minerals forming from submarine weathering of igneous rocks such as basalts. Water is also consumed in these reactions, accounting for minor increases in total interstitial salinity. Terrigenous, organic-rich sediments deposited rapidly along continental margins also exhibit significant evidences of alteration. Microbial reactions involving organic matter lead to complete removal of SO4(post staggered 2-), strong HCO3(post staggered -) enrichment, formation of NH4(post staggered +), and methane synthesis from H2 and CO2 once SO4(post staggered 2-) is eliminated. K+ and often Na+ (slightly) are depleted in the interstitial waters. Ca(2+) depletion may occur owing to precipitation of CaCO3. In most cases interstitial Cl- remains relatively constant, but increases are noted over evaporitic strata, and decreases in interstitial Cl- are observed in some sediments adjacent to continents.

Chemical and isotopic compositions of deep-sea carbonate sediments and interstitial waters from DSDP Sites 30-288 and 30-289

Interstitial waters and sediments from DSDP sites 288 and 289 contain information on the chemistry and diagenesis of carbonate in deep-sea sediments and on the role of volcanic matter alteration processes. Sr/Ca ratios are species dependent in unaltered foraminifera from site 289 and atom ratios (0.0012-0.0016) exceed those predicted by distribution coefficent data (~0.0004). During diagenesis Sr/Ca ratios of carbonates decrease and reach the theoretical distribution at a depth which is identical to the depth of Sr isotopic equilibration, where 87Sr/86Sr ratios of interstitial waters and carbonates converge. Mg/Ca ratios in the carbonates do not increase with depth as found in some other DSDP sites, possibly because of diagenetic re-equilibration with interstitial waters showing decreasing Mg(2+)/Ca(2+) ratios with depth due to Ca input and Mg removal by alteration of volcanic matter. Interstitial 18O/16O ratios increase with depth at site 289 to d18O = 0.67? (SMOW), reflecting carbonate recrystallization at elevated temperatures (>/= 20°C), the first recorded evidence of this effect in interstitial waters. Interstitial Sr2+ concentrations reach high levels, up to 1 mM, chiefly because of carbonate recrystallization. However, 87Sr/86Sr ratios decrease from 0.7092 to less than 0.7078, lower than for contemporaneous sea water, showing that there is a volcanic input of strontium at depth. This volcanic component is recorded in the Sr isotopic composition of recrystallized calcites. Isotopic compositions of the unrecrystallized calcites suggests that the rate of increase of the 87Sr/86Sr ratio of sea water with time has been faster since 3 my ago than in the preceding 13 my.

Pollen and non-pollen palynomorph records of the Holocene part of the composite sediment core TMD from lake Tso Moriri (NW India) and of modern surface samples from the Tso Moriri region

The high-altitude lake Tso Moriri (32°55'46'' N, 78°19'24'' E; 4522 m a.s.l.) is situated at the margin of the ISM and westerly influences in the Trans-Himalayan region of Ladakh. Human settlements are rare and domestic and wild animals are concentrating at the alpine meadows. A set of modern surface samples and fossil pollen from deep-water TMD core was evaluated with a focus on indicator types revealing human impact, grazing activities and lake system development during the last ca. 12 cal ka BP. Furthermore, the non-pollen palynomorph (NPP) record, comprising remains of limnic algae and invertebrates as well as fungal spores and charred plant tissue fragments, were examined in order to attest palaeolimnic phases and human impact, respectively. Changes in the early and middle Holocene limnic environment are mainly influenced by regional climatic conditions and glacier-fed meltwater flow in the catchment area. The NPP record indicates low lake productivity with high influx of freshwater between ca. 11.5 and 4.5 cal ka BP which is in agreement with the regional monsoon dynamics and published climate reconstructions. Geomorphologic observations suggest that during this period of enhanced precipitation the lake had a regular outflow and contributed large amounts of water to the Sutlej River, the lower reaches of which were integral part of the Indus Civilization area. The inferred minimum fresh water input and maximum lake productivity between ca. 4.5-1.8 cal ka BP coincides with the reconstruction of greatest aridity and glaciation in the Korzong valley resulting in significantly reduced or even ceased outflow. We suggest that lowered lake levels and river discharge on a larger regional scale may have caused irrigation problems and harvest losses in the Indus valley and lowlands occupied by sedentary agricultural communities. This scenario, in turn, supports the theory that, Mature Harappan urbanism (ca. 4.5-3.9 cal ka BP) emerged in order to facilitate storage, protection, administration, and redistribution of crop yields and secondly, the eventual collapse of the Harappan Culture (ca. 3.5-3 cal ka BP) was promoted by prolonged aridity. There is no clear evidence for human impact around Tso Moriri prior to ca. 3.7 cal ka BP, with a more distinct record since ca. 2.7 cal ka BP. This suggests that the sedimentary record from Tso Moriri primarily archives the regional climate history.

Isotopic analyses of DSDP Hole 45-395A samples from veins containing gypsum (Table 1)

Gypsum and halite crystals, together with saponite and phillipsite, were found in a vein in a basalt sill 625 m below the sea floor at DSDP Site 395A, located 190 km west of the crest of the Mid-Atlantic Ridge. The delta34S value of the gypsum (+19.4?) indicates a seawater source for the sulfate. The delta18O values of the saponite (+19.9?) and phillipsite (+18.1?) indicate either formation from normal seawater at about 55°C or formation from delta18O-depleted seawater at a lower temperature. The gypsum (which could be secondary after anhydrite) was formed by reaction between Ca[2+] released from basalt and SO4[2-] in circulating seawater. The halite could have formed when water was consumed by hydration of basalt under conditions of extremely restricted circulation. A more probable mechanism is that the gypsum was originally precipitated as anhydrite at temperatures above 60°C. As the temperature dropped the anhydrite converted to gypsum. The conversion would consume water, which could cause halite precipitation, and would cause an increase in the volume of solids, which would plug the vein and prevent subsequent dissolution of the halite.

Biogenic silica and silicic acid benthic fluxes of a North-eastern Atlantic Abyssal Locality

Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 µM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment-water interface to <1% at depth. Benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non-linear DSi porewater gradients. Some 90% of the dissolution occurred within the top 5.5 cm of the sediment column, rather than at the sediment-water interface and the annual DSi efflux was close to 0.057 mol Si/m**2/yr. Biogenic silica accumulation was close to 0.008 mol Si/m**2/yr and the annual opal delivery reconstructed from sedimentary fluxes, assuming steady state, was 0.065 mol Si/m**2/yr. This is in good agreement with the mean annual opal flux determined from sediment trap samples, averaged over the last decade (0.062 mol Si/m**2/yr). Thus ca. 12% of the opal flux delivered to the seafloor get preserved in the sediments. A simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5.5 cm of the sediment column suggests that there should be no accumulation of opal in PAP sediments. However, by combining the BENGAL high sampling frequency with our experimental results on BSi dissolution, we conclude that non-steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of BSi in these sediments. This comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor. Hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non-local mixing events by ensuring the rapid transport of BSi particles deep into the sediment to where saturation is reached.

Skeletal density of Porites

Paleotemperature estimates based on coral Sr/Ca have not been widely accepted because the reconstructed glacial-Holocene shift in tropical sea-surface temperature (~4-6°C) is larger than that indicated by foraminiferal Mg/Ca (~2-4°C). We show that corals over-estimate changes in sea-surface temperature (SST) because their records are attenuated during skeletogenesis within the living tissue layer. To quantify this process, we microprofiled skeletal mass accumulation within the tissue layer of Porites from Australasian coral reefs and laboratory culturing experiments. The results show that the sensitivity of the Sr/Ca and d18O thermometers in Porites will be suppressed, variable, and dependent on the relationship between skeletal growth rate and mass accumulation within the tissue layer. Our findings help explain why d18O-SST sensitivities for Porites range from -0.08 per mil/°C to -0.22 per mil/°C and are always less than the value of -0.23 per mil/°C established for biogenic aragonite. Based on this observation, we recalibrated the coral Sr/Ca thermometer to determine a revised sensitivity of -0.084 mmol/mol/°C. After rescaling, most of the published Sr/Ca-SST estimates for the Indo-Pacific region for the last ~14,000 years (-7°C to +2°C relative to modern) fall within the 95% confidence envelope of the foraminiferal Mg/Ca-SST records. We conclude that two types of calibration scales are required for coral paleothermometry; an attenuated Porites-specific thermometer sensitivity for studies of seasonal to interannual change in SST and, importantly, the rescaled -0.084 mmol/mol/°C Sr/Ca sensitivity for studies of 20th-century trends and millennial-scale changes in mean SST. The calibration-scaling concept will apply to the development of transfer functions for all geochemical tracers in corals.

Viral decay and production in sediments of the Mediterranean Sea

Here, for the first time, we have carried out synoptic measurements of viral production and decay rates in continental-shelf and deep-sea sediments of the Mediterranean Sea to explore the viral balance. The net viral production and decay rates were significantly correlated, and were also related to prokaryotic heterotrophic production. The addition of enzymes increased the decay rates in the surface sediments, but not in the subsurface sediments. Both the viral production and the decay rates decreased significantly in the deeper sediment layers, while the virus-to-prokaryote abundance ratio increased, suggesting a high preservation of viruses in the subsurface sediments. Viral decay did not balance viral production at any of the sites investigated, accounting on average for c. 32% of the gross viral production in the marine sediments. We estimate that the carbon (C) released by viral decay contributed 6-23% to the total C released by the viral shunt. Because only ca. 2% of the viruses produced can infect other prokaryotes, the majority is not subjected to direct lysis and potentially remains as a food source for benthic consumers. The results reported here suggest that viral decay can play an important role in biogeochemical cycles and benthic trophodynamics.

Geochemistry and age model of sediment core BDP96-1

Within the framework of the Baikal Drilling Project (BDP), a 192 m long sediment core (BDP-96-1) was recovered from the Academician Ridge, a submerged topographic high between the North and Central Basins of Lake Baikal. Sedimentological, clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth, corresponding to ca. 2.4-3.4 Ma. The aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of Northern Hemisphere glaciation in Late Pliocene time. A major climate change occurred in the Lake Baikal area at about 2.65 Ma. Enhanced physical weathering in the catchment, mirrored in the illite to smectite ratio, and temporarily reduced bioproduction in the lake, reflected by the diatom abundance, evidence a change towards a colder and more arid climate, probably associated with an intensification of the Siberian High. In addition, the coincident onset of distinct fluctuations in these parameters and in the Zr/Al ratio suggests the beginning of the Late Cenozoic high amplitude climate cycles at about 2.65 Ma. Fluctuations in the Zr/Al ratio are traced back to changes in the aeolian input, with high values in warmer, more humid phases due to a weaker Siberian High. Assuming that the sand content in the sediment reflects tectonic pulses, the Lake Baikal area was tectonically active during the entire investigated period, but in particular around 2.65 Ma. Tectonic movements have likely led to a gradual catchment change since about 3.15 Ma from the western towards the eastern lake surroundings, as indicated in the geochemistry and clay mineralogy of the sediments. The strong coincidence between tectonic and climatic changes in the Baikal area hints at the Himalayan uplift being one of the triggers for the Northern Hemisphere Glaciation.

Planktonic foraminiferal preservation in Plio/Pleistocene sediments from the western equatorial Atlantic and Caribbean

Records of mean sortable silt and planktonic foraminiferal preservation from the Ceará Rise (western equatorial Atlantic) and from the Caribbean are presented to analyze the Pliocene (3.5-2.2 Ma) to Pleistocene (1.6-0.3 Ma) evolution of near-bottom current strength and the carbonate corrosiveness of deep water. During the mid-Pleistocene climate transition (~1 Ma) a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered, demonstrating a substantial decrease in the glacial contribution of the Lower North Atlantic Deep Water (LNADW) to the Atlantic Ocean. Also, an increased sensitivity to eccentricity orbital forcing is registered after the MPT. By contrast, carbonate preservation increases considerably in the deep Caribbean in response to a strong and persistent stable contribution of Upper North Atlantic Deep Water (UNADW). We found evidence for the strongest and most stable circulation within the LNADW cell during the Northern Hemisphere cooling period between ~3.2 and 2.75 Ma. This is in agreement with the 'superconveyor model' which postulates that the highest NADW production took place prior to ~2.7 Ma. A considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large-scale continental ice sheets in the Northern Hemisphere. This documents the final termination of the 'superconveyor' at ca. 2.75 Ma. However, our data do not support a 'superconveyor' in the interval between 3.5 and 3.2 Ma when high-amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed. Because of the great sensitivity of NADW production to changes in surface water salinity, we assume that the high-amplitude fluctuations of LNADW circulation prior to ~3.2 Ma are linked to changes in the Atlantic salinity budget. After 2.75 Ma they are primarily controlled by ice-sheet forcing. In contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep Atlantic, a trend toward better preservation from the Pliocene to Pleistocene is observed in the deep Caribbean. This indicates a long-term increase in the contribution of UNADW to the Atlantic Ocean.

Interstitial water chemistry of DSDP Leg 85 sites

The concentration changes in pore waters of dissolved calcium, magnesium, sulfate, strontium, and silica and of alkalinity are controlled by diagenetic reactions occurring within the biogenic sediments of DSDP Sites 572, 573, and 574. Downcore increases in dissolved Sr2 + indicate recrystallization of calcite, and increases in dissolved SiO2 reflect dissolution of amorphous silica. Minor gradients in dissolved Ca(2+) and Mg(2+) suggest little if any influence from reactions involving volcanic sediments or basalt. Differences in interstitial water profiles showing the downhole trends of these chemical species mark variations in carbonate and silica diagenesis, sediment compositions, and sedimentation rate histories among the sites. The location and extent of carbonate diagenesis in these sediments are determined from Sr/Ca distributions between the interstitial waters and the bulk carbonate samples. Pore water strontium increases in the upper 100 to 250 m of sediment are assumed to reflect diffusion from underlying zones where calcite recrystallization has occurred. On the basis of calculations of dissolved strontium production and comparisons between observed and calculated "equilibrium" Sr/Ca ratios of the solids, approximately 30 to 50% of the carbonate has recrystallized in these deeper intervals. These estimates agree with the observed amounts of chalk at these sites. Variations in Sr/Ca ratios of these carbonates reflect differences in calcareous microfossil content, in diagenetic history, and, possibly, in changes in seawater Sr/Ca with time. Samples of porcelanite recovered below 300 m at Site 572 suggest formation at temperatures 20 to 30° C greater than ones estimated assuming oceanic geothermal gradients from sedimentary sections similar to those recovered on Leg 85. The higher temperatures may partially account for higher Sr/Ca ratios determined for recrystallized carbonates from this site.

(Table 1) Pollen analysis of clay-gyttja layers obtained in 1966 near Hitzhusen, north Germany

Bereits im Jahre 1956 wurde bei Baugrund-Aufschlußbohrungen für das zweite Kurmittelhaus in Bad Bramstedt bei einer Serie von 11 Bohrungen - ausgeführt durch die Firma Fritz Eising K. G. in Hamburg - in drei benachbart gelegenen Bohrpunkten an der südlichen Ecke des Gebäudes in einer Teufe von ca. 10 m u. T. eine offensichtlich organogene Schicht von ca. 2 m Mächtigkeit erbohrt. Eines dieser Bohrprofile hat folgenden Aufbau: -5,8 m Fein-Mittelsand, -7,7 m Mittelsand, Fein-Mittelkies, -10,0 m Mittelsand, wenig Kies, -12,0 m Gyttja, -15,0 m Mittelsand, Grobsand. Die bereits wiedergegebene Teufenangabe ist insofern recht interessant, als im Jahre 1966 bei der Brücke über die Bramau bei Hitzhusen, demnach in der Talaue der Bramau in einer Teufe von 8,55 m ebenfalls eine Gyttja erbohrt wurde. Die Tiefenlagen beider Vorkommen scheinen sich demnach relativ zu entsprechen. Das gesamte Profil bei Hitzhusen ist in einigen Punkten abweichend ausgebildet und enthält vor allem noch ein zweites Gyttja-Band in 11,25 m Teufe. Im Einzelnen wurde hier durch die Bohrfirma Paul Hammers A. G., Hamburg, diese Schichtfolge angetroffen: -1.55 m Fein-Mittelsand, Humus, -3,10 m Mittel-Grobsand, Kies, Steine, etwas Lehm, -4,50 m Mittel-Grobsand, -7,20 m Mittel-Grobsand, Kies, -8,00 m Grobsand, -8,55 m Grobsand, Kies, -8,65 m Schluff-Gyttja, -9,70 m Fein-Grobsand, -10,25 m Mittel-Grobsand, Kies, -10,75 m Mittel-Grobsand, -11,25 m Mittel-Grobsand, Schluffstreifen, -11,40 m Schluff-Gyttja, -12,10 m Mittelsand, -12,30 m Mittel-Grobsand, Kies, -17,85 m Geschiebemergel. Die gewonnenen Proben der Schluff-Gyttjen wurden näher untersucht. Da es sich in beiden Fällen um geringmächtige Lagen handelt (0,1 m resp. 0,15 m), und das Material durchaus als stark feinsandig bis schluffig zu bezeichnen ist (das spricht für eine wesentlich schnellere Sedimentation, als die einer reinen biogenen Gyttja), ist der Effekt einer 'Mischprobe' weitgehend ausgeschlossen. Außerdem lagen die Proben - obgleich wahrscheinlich mit einem Ventilbohrer gefördert - als relativ ungestörte Brocken mit erhaltengebliebener Feinschichtung vor. Auf den Schichtflächen waren gröbere Pflanzenreste erkennbar (in der Tabelle angegeben). Der sehr hohe mineralische Anteil läßt zunächst den Verdacht auf sekundären Pollen aufkommen. Keines der beiden pollenanalytisch ermittelten Vegetationsbilder liefert dagegen irgendeine Bestätigung hierfür.

(Table 1) Summary statistics of taxa in samples of ODP Hole 181-1123B

Planktonic foraminifera are used to identify late Pliocene-Quaternary near surface water masses on the northeastern flank of Chatham Rise by comparison with faunas in core-tops east of New Zealand. In an overview study, distance measures, ordinations, and discriminant analysis are applied to 32 faunas from Site 1123B to identify similar faunas among 35 core-tops between 35 and 61°S east of New Zealand. Many Site 1123B faunas in the 2.72 myr interval sampled compare with those in core-tops on the northern side of Chatham Rise from a similar latitude, and are identified as transitional zone assemblages now associated with the subtropical gyre. This result is consistent with studies of late Quaternary planktonic foraminifera from this region and suggests that, typically, the Subtropical Front was locked to Chatham Rise through glacial and interglacial periods, at least back to the late Pliocene. However, a fauna at ca. 1.17 Ma compares with subpolar assemblages in core-tops between 44 and 48°S and identifies cooler surface water. Expectedly, closer sampling may reveal additional periods when southern water moved over the northeastern flank of Chatham Rise. Although the dominance of Globorotalia inflata, a species typical of the southern margin of subtropical gyres, is a principal feature of Site 1123B faunas, in a minority it is replaced as the most abundant species by dextral populations of Neogloboquadrina pachyderma, particularly about the time of the middle Pleistocene transition. Close analogues of these variant transitional assemblages are not present in core-tops about Chatham Rise but sediment trap and coretop data from other regions suggest that they identify high fertility in the mixed layer associated with upwelling or mixing of water masses. The proportion of sinistrally coiled Neogloboquadrina pachyderma rises to ca. 0.6 between ca. 2.45 and 2.57 Ma, soon after the intensification of Northern Hemisphere glaciation. Although the coiling data indicate subantarctic near surface water, the species remains rare. As the faunas retain their transitional zone character, only minor entrainment of subantarctic water may have occurred.

Sedimentology on a core from Lonar Lake in India

Concerns about the regional impact of global climate change in a warming scenario have highlighted the gaps in our understanding of the Indian Summer Monsoon (ISM, also referred to as the Indian Ocean summer monsoon) and the absence of long term palaeoclimate data from the central Indian core monsoon zone (CMZ). Here we present the first high resolution, well-dated, multiproxy reconstruction of Holocene palaeoclimate from a 10 m long sediment core raised from the Lonar Lake in central India. We show that while the early Holocene onset of intensified monsoon in the CMZ is similar to that reported from other ISM records, the Lonar data shows two prolonged droughts (PD, multidecadal to centennial periods of weaker monsoon) between 4.6-3.9 and 2-0.6 cal?ka. A comparison of our record with available data from other ISM influenced sites shows that the impact of these PD was observed in varying degrees throughout the ISM realm and coincides with intervals of higher solar irradiance. We demonstrate that (i) the regional warming in the Indo-Pacific Warm Pool (IPWP) plays an important role in causing ISM PD through changes in meridional overturning circulation and position of the anomalous Walker cell; (ii) the long term influence of conditions like El Niño-Southern Oscillation (ENSO) on the ISM began only ca. 2 cal?ka BP and is coincident with the warming of the southern IPWP; (iii) the first settlements in central India coincided with the onset of the first PD and agricultural populations flourished between the two PD, highlighting the significance of natural climate variability and PD as major environmental factors affecting human settlements.

Interstitial water chemistry at DSDP Leg 79 Holes

Interstitial water analyses of samples collected at Sites 544-547 of DSDP Leg 79 are presented. In Site 547 chloride concentrations increase to almost 80% of the halite saturation values. Gypsum occurrences in the sediments immediately overlying the halite deposit can be explained in terms of migration of Ca**2+ and SO2**2- from the underlying evaporites. At shallower depths sulfate concentrations decrease rapidly as a result of sulfate reduction processes. The same processes lead to the removal of calcium in the form of calcium carbonate. At Site 547, the chloride concentration depth profile suggests a maximum of dissolved chloride which may be the result of advective flow from nearby (abput 6 km) evaporite salt diapirs.

Long chain alkyl diol distribution in Arabian Sea surface sediments

Oxygen exposure has a large impact on lipid biomarker preservation in surface sediments and may affect the application of organic proxies used for reconstructing past environmental conditions. To determine its effect on long chain alkyl diol and keto-ol based proxies, the distributions of these lipids was studied in nine surface sediments from the Murray Ridge in the Arabian Sea obtained from varying water depths (900 to 3000 m) but in close lateral proximity and, therefore, likely receiving a similar particle flux. Due to substantial differences in bottom water oxygen concentration (<3 to 77 µmol/L) and sedimentation rate, substantial differences exist in the time the biomarker lipids are exposed to oxygen in the sediment. Long chain alkyl diol and keto-ol concentrations in the surface sediments (0-0.5 cm) decreased progressively with increasing oxygen exposure time, suggesting increased oxic degradation. The 1,15-keto-ol/diol ratio (DOXI) increased slightly with oxygen exposure time as diols had apparently slightly higher degradation rates than keto-ols. The ratio of 1,14- vs. 1,13- or 1,15-diols, used as upwelling proxies, did not show substantial changes. However, the C30 1,15-diol exhibited a slightly higher degradation rate than C28 and C30 1,13-diols, and thus the Long chain Diol Index (LDI), used as sea surface temperature proxy, showed a negative correlation with the maximum residence time in the oxic zone of the sediment, resulting in ca. 2-3.5 °C change, when translated to temperature. The UK'37 index did not show significant changes with increasing oxygen exposure. This suggests that oxic degradation may affect temperature reconstructions using the LDI in oxic settings and where oxygen concentrations have varied substantially over time.