Sea surface temperature reconstruction based on alkenones of sediment core M35003-4

Evidence for abrupt climate changes on millennial and shorter timescales is widespread in marine and terrestrial climate records (Dansgard et al., 1993, doi:10.1038/364218a0; Bond et al., 1993, doi:10.1038/365143a0; Charles et al., 1996, doi:10.1016/0012-821X(96)00083-0, Bard et al., 1997, doi:10.1038/385707a0). Rapid reorganization of ocean circulation is considered to exert some control over these changes (Broecker et al., 1985, doi:10.1038/315021a0), as are shifts in the concentrations of atmospheric greenhouse gases (Broecker, 1994, doi:10.1038/372421a0). The response of the climate system to these two influences is fundamentally different: slowing of thermohaline overturn in the North Atlantic Ocean is expected to decrease northward heat transport by the ocean and to induce warming of the tropical Atlantic (Crowley, 1992, doi:10.1029/92PA01058; Manabe and Stouffer, 1997, doi:10.1029/96PA03932), whereas atmospheric greenhouse forcing should cause roughly synchronous global temperature changes (Manabe et al., 1991, doi:10.1175/1520-0442(1991)004<0785:TROACO>2.0.CO;2). So these two mechanisms of climate change should be distinguishable by the timing of surface-water temperature variations relative to changes in deep-water circulation. Here we present a high-temporal-resolution record of sea surface temperatures from the western tropical North Atlantic Ocean which spans the past 29,000 years, derived from measurements of temperature-sensitive alkenone unsaturation in sedimentary organic matter. We find significant warming is documented for Heinrich event H1 (16,900-15,400 calendar years bp) and the Younger Dryas event (12,900-11,600 cal. yr bp), which were periods of intense cooling in the northern North Atlantic. Temperature changes in the tropical and high-latitude North Atlantic are out of phase, suggesting that the thermohaline circulation was the important trigger for these rapid climate changes.

Chemistry of Late Quaternary sediments and their interstitial waters of sediment cores from the Nort-West African continental margin

In the sediments of the NW African continental margin the mainly biogenic carbonate constituents become increasingly diluted with terrigenous material as one approaches the coast, as indicated by the carbonate-CO2 content, the Al2O3/SiO2-ratios, and the presence of ammonia fixed to alumino-silicates, predominantly to illites. In the norther area of the investigation - off Cape Blanc and Cape Bojador . the terrigenous constituents are mainly quartz from the Sahara Desert, whereas in the south - off Senegal - more alumino-silicates as clay minerals are admixed with the carbonate constituents. The organic carbon content of the continental slope sediments off Senegal is higher than in samples of the continental rise or of the preservation of organic matter as a result of high production and relatively rapid sedimentation. The zone of manganese-oxide enrichment follows the redox potential of + 330 mV from the surface (0-5 cm) into the sediments (20-30 cm deep) at 2000--3000 m and 3700 m of water depths, respectively. At shallower water depths, low redox potentials preclude deposition of manganese oxides and cause their mobilization from the sediments. About 1/3 of the total sedimentary Zn and 1/4 of the Cu is associated with the carbonate mineral fraction, probably in calcium phosphate overgrowths as a result of the mineralization of phosphorus-containing organic matter. Besides the precipitation of calcium phosphate, the mineralization of organic matter mediated by bacterial sulfate reduction also results in calcium carbonate precipitation and the exchange of ammonia for potassium on illites. Because of these simultaneous reactions, the depth distribution of all mineralization constituents in the interstitial water can be determined using the actual molar carbon-to-nitrogen-to phosphorus ratios of the sedimentary organic matter. The amount of sulfide sulfur in this process indicates the predominance of bacterial sulfate reduction in the sediments off NW Africa. This process also preferentially decomposes nitrogen- and phosphorus-containing organic compounds so organic matter deficient in these elements is characteristic for the rapidly accumulating sediments than today, indicating there was increased production of organic carbon compounds and more favorable conditions of their preservations. During the last interglacial times conditions were similar to those to today. This differentiation with time has also been observed in sediments from the Argentine Basin and from slope off South India indicating perhaps world-wide environmental changes throughout Late Quaternary times.

Geochemistry of recent marine sediments, interstitial water, and sea water from the West African continental margin

Carbon dioxide, ammonia, and reactive phosphate in the interstitial water of three sediment cores of the West African continental margin result from oxidation of sedimentary organic matter by bacterial sulfate reduction. The proposed model is a modification of one initially suggested by Richards (1965) for processes in anoxic waters: (CH2O)106 (NH3)8 (H3PO4) (0.7-0.2) + 53 SO4**2- =106 CO2 + 106 H20 + 8 NH3 + (0.7 - 0.2) H3PO4 + 53 S**2- The amount of reduced interstitial sulfate, the carbon-to-nitrogen-to-phosphorus atomic ratio of the sedimentary organic matter, as well as small amounts of carbon dioxide, which precipitated as interstitial calcium carbonate, are included in the general oxidation-reduction reaction. Preferential loss of nitrogen and phosphorus from organic matter close to the surface was recorded in both the interstitial water and sediment composition. It appeared that in deeper sections of the core organic carbon compounds were oxidized which were probably in an even lower oxidation state than that indicated by the proposed model. An estimated 2 % of the amount of organic matter still present was oxidized after it became incorporated into the sediment; whereas sulfide sulfur contents indicate that a much larger percentage (15-20%) seemed to have been subject to bacterial oxidation during the Pleistocene period, when a very thin oxidizing layer on the sediment allowed the above decomposition process to start relatively early favoured by almost fresh organic matter, and by almost unrestricted exchange of sulfate with the overlying water.

(Table 1) Composition of pore water from ODP Site 997 holes A and B

Pore water extracted from sediments penetrated on Leg 164 of the Ocean Drilling Program at the Blake Ridge West. Atlantic were analyzed for acetate, total dissolved organic carbon, bromide and iodide, to help explain the occurrence of subsurface maxima in bacteria biomass and activity reported previously from a nearby site. The high concentrations of these organic matter decomposition by-products in the pore waters from sediments with moderate concentrations of sedimentary organic matter can convincingly be modelled as resulting from upward migration of pore water. The amount of acetate and unidentified DOC transported by the pore water contribute significantly to the pool of metabolizable carbon, and may be the most important substances in energetic terms.

Concentrations and isotopic compositions of low molecular weight hydrocarbons in hydrothermal fluids from ODP Field, Middle Valley, northern Juan de Fuca Ridge

The presence of sedimentary organic matter blanketing midocean ridge crests has a potentially strong impact on metal transport in hydrothermal vent fluids. To constrain the role of organic matter in metal mobility during hydrothermal sediment alteration, we reacted organic-rich diatomaceous ooze from Guaymas Basin, Gulf of California, and organic-poor hemipelagic mud from Middle Valley, northern Juan de Fuca Ridge, with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity, at 275° to 400°C, 350 to 500 bars, and initial fluid: sediment mass ratios ranging from 1.6 to 9.8. Reaction of these fluids with both sediment types released CO2 and high concentrations of ore-forming metals (Fe, Mn, Zn, Pb) to solution. Relatively low concentrations of Cu were observed in solution and likely reflect the reducing conditions that resulted from the presence of sedimentary organic matter. Both the concentrations of CO2 and dissolved metals were lower in fluids reacted with Middle Valley sediment compared with aqueous concentrations in fluids reacted with Guaymas Basin sediment. During alteration of both sediment types, metal concentrations varied strongly as a function of temperature, increasing by up to an order of magnitude over the 75°C range of each experiment. Major element fluid chemistry and observed alteration assemblages suggest that during hydrothermal alteration of organic-lean sediment from Middle Valley a feldspar-quartz-illite mineral assemblage buffered in situ pH. In contrast, data from the experimental alteration of organic-rich Guaymas Basin sediment suggest that a calcite-plagioclase-quartz assemblage regulated in situ pH. Fluid speciation calculations suggest that in situ pH during Guaymas Basin sediment alteration was lower than during alteration of Middle Valley sediment and accounts for the substantially greater metal mobility at a given temperature and pressure during the former experiment. Comparison of our results with the results of basalt alteration experiments indicate that except for Cu, hydrothermal sediment alteration results in equal or greater concentrations of ore-forming metals at a given temperature and pressure. Accordingly, the presence of ore-forming metals in fluids currently venting from sediment-covered hydrothermal systems at concentrations substantially lower than in fluids from bare-rock systems may reflect chemical reequilibration during subsurface cooling within the sediment pile.

(Table 1) Age determination of sediment core AAS-62/2

Variations in the Indian summer monsoon (ISM) intensity during the last 16.7?ka have been studied using organic carbon (Corg), d15N of sedimentary organic matter, CaCO3, sediment texture, d18OC, and Mg/Ca-derived sea surface temperature, d18O of sea water and sea surface salinity, in a 14C-dated sediment core from the eastern Arabian Sea. The d18O in water and planktonic foraminifera shells off the central west coast of India may be controlled by the ISM intensity as this area receives high precipitation and land runoff. Also, the Corg and CaCO3 contents of sediments and d15N of sedimentary organic matter may be linked to ISM-induced productivity and denitrification. The results of the present study reveal that between 16 and 15.2 ka BP, the ISM was weak with minor fluctuations and started intensifying around 15.2 ka BP, at the onset of the Bølling-Ållerød (B-A) event. The B-A event is characterized by higher water column denitrification rates comparable to the present day. The ISM signatures observed in the d18OC record of B-A event compare well with those from Timta cave of the western Himalayas and also the Asian summer monsoon signatures from the Hulu caves in China and warming signatures in Greenland Ice Sheet Project 2 (GISP2) suggesting atmospheric teleconnections through Intertropical Convergence Zone. The boundary between the Younger Dryas and the Holocene is discernible with small episodes of abrupt events of increased ISM intensity. This decrease in d18OC values at ~11.8 ka BP is contemporary with June solar insolation maximum at 30° north and the increase in methane in the GISP2 ice core supporting episodes of warmer climate and increase in ISM intensity. The ISM seems to have been most stable between 7 and 5.6 ka BP. The core exhibits periodicity of 500 years that is comparable to the Atlantic water formation and the Chinese monsoon.

Geochemistry of carbon at DSDP Legs 58 and 59 Holes

I obtained 68 quarter sections of cores from the JOIDES Organic Geochemistry Panel for studying type, distribution, and stages of organic diagenesis of sedimentary organic matter in the West Philippine and Parece Vela basins and Mariana Trough area (Figure 1). The present chapter compares (1) 11 geochemical parameters used to determine organic source and its stage of genesis within the 9 site locations in this study area and (2) compares these 11 with the same parameters reported from Leg 56, outer trench slope of the Japan Trench, and Leg 60, Mariana Trough and Trench (Schorno, in press a, b). Even though these sediments are considered pelagic, the organic content in most of the core sections appears to be hemipelagic. The sedimentary organic matter in these cores is believed to be in an early stage of diagenesis. Both conclusions are based primarily on the fl-alkane distribution within the organic matter. This particular parameter, I note later, has a major weakness. As Hunt (Hunt, 1979) and I (in press b) observed, marine organisms synthesize /z-alkanes with distributions containing neither odd nor even preferences. Thus those sediments that did contain w-alkane distributions with OEP near 1, suggesting a late stage of catagenesis, may in actuality be immature marine sediments.

(Table 1) Stable nitrogen isotopes of ammonium in interstitial waters from ODP Site 164-997

Ammonium (NH4+) concentration profiles in piston-core sediments of the Carolina Rise and Blake Ridge generally have linear concentration profiles within the sulfate reduction zone (Borowski, 1998). Deep Sea Drilling Project (DSDP) Site 533, located on the Blake Ridge, also displayed a linear ammonium concentration profile through the sulfate reduction zone and the profile linearity continues into the upper methanogenic zone to a depth of ~200 meters below seafloor (mbsf), where the first methane gas hydrates probably occur (Jenden and Gieskes, 1983, doi:10.2973/dsdp.proc.76.114.1983; Kvenvolden and Barnard, 1983, doi:10.2973/dsdp.proc.76.106.1983). Sediments from the Ocean Drilling Program (ODP) Leg 164 deep holes (Sites 994, 995, and 997) also exhibit linear ammonium profiles above the top of the gas hydrate zone (~200 mbsf) (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996). We hypothesized that a possible cause of linear ammonium profiles was diffusion of ammonium from a concentrated ammonium source at depth. We further reasoned that if this ammonium were produced by microbial fermentation reactions at depth, that a comparison of the nitrogen isotopic composition of sedimentary organic nitrogen and the nitrogen with pore-water ammonium would test this hypothesis. Convergence with depth of d15N values of the nitrogen source (sedimentary organic matter) and the nitrogen product (dissolved NH4+) would strongly suggest that ammonium was produced within a particular depth zone by microbial fermentation reactions. Here, we report d15N values of pore-water ammonium from selected interstitial water (IW) samples from Site 997, sampled during ODP Leg 164.

Alkenone unsaturation index and alkenones in ODP Hole 161-977A

A number of essential elements closely related to each other are involved in the Earth's climatic system. The temporal and spatial distribution of insolation determines wind patterns and the ocean's thermohaline pump. In turn, these last two are directly linked to the extension and retreat of marine and continental ice and to the chemistry of the atmosphere and the ocean. The variability of these elements may trigger, amplify, sustain or globalize rapid climatic changes. Paleoclimatic oscillations have been identified in this thesis by using fossil organic compounds synthesized by marine and terrestrial flora. High sedimentation rate deposits at the Barents and the Iberian peninsula continental margins were chosen in order to estimate the climatic changes on centennial time resolution. At the Barents margin, the sediment recovered was up to 15,000 years old (unit ''a'', from latin ''annos'') (M23258; west of the Bjørnøya island). At the Iberian margin, the sediment cores studied covered a wide range of time spans: up to 115,000 a (MD99-2343; north of the Minorca island), up to 250,000 a (ODP-977A; Alboran basin) and up to 420,000 a (MD01-2442, MD01-2443, MD01-2444, MD01-2445; close to the Tagus abyssal plain). At the northern site, inputs containing marine, continental and ancient reworked organic matter provided a detailed reconstruction of climate history at the time of the final retreat of the Barents ice sheet. At the western Barents continental slope, warm climatic conditions were observed during the early Holocene (~from 8,650 a to 5,240 a ago); in contrast, an apparent long-term cooling trend occurred in the late Holocene (~from 5,240 a to 760 a ago), in consistence with other paleoarchives from northern and southern European latitudes. The Iberian margin sites, which were never covered with large ice sheets, preserved exceptionally complete sequences of rapid events during ice ages hitherto not studied in such great detail: during the last glacial (~from 70,900 a to 11,800 a ago), the second glacial (~from 189,300 a to 127,500 a ago), the third ice age (~from 278,600 a to 244,800 a ago) and the fourth (~from 376,300 a to 337,500 a ago). In this thesis, crucial research questions were brought up concerning the severity of different glacial periods, the intensity and rates of the recorded oscillations and the long distance connections related to rapid climate change. The data obtained provide a sound basis to further research on the mechanisms involved in this rapid climate variability. An essential point of the research was the evidence that, over the past 420,000 a, at the whole Iberian margin, warm and stable long periods similar to the Holocene always ended abruptly in few centuries after a gradual deterioration of climate conditions. The detailed estimate of past climate variability provides clues to the natural end of the present warm period. Returning to an ice age in European lands would be exacerbated by a number of factors: a lack of differential solar heating between northern and southern north Atlantic latitudes, enhanced evaporation at low latitudes, and an increase in snowfall or iceberg discharges at northern regions. It must be emphasized that all climatic oscillations observed in this thesis were caused by forces of nature, i.e. the last two centuries were not taken into consideration.