Multi-proxy analyses of sediment cores from the Pakistan continental margin

Late Holocene laminated sediments from a core transect centred in the oxygen minimum zone (OMZ) impinging at the continental slope off Pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years. High SW-monsoon-controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon (TOC) and redox-sensitive elements (Ni, V), as well as by a low-diversity, high-abundance benthic foraminiferal Buliminacea association and high abundance of the planktonic species Globigerina bulloides indicative of upwelling conditions. Surface-water productivity was strongest during SW monsoon maxima. Stable OMZ conditions (reflected by laminated sediments) were found also during warm interstadial events (Preboreal, Bølling-Allerød, and Dansgaard-Oeschger events), as well as during peak glacial times (17-22.5 ka, all ages in calendar years). Sediment mass accumulation rates were at a maximum during the Preboreal and Younger Dryas periods due to strong riverine input and mobilisation of fine-grained sediment coinciding with rapid deglacial sea-level rise, whereas eolian input generally decreased from glacial to interglacial times. In contrast, the occurrence of bioturbated intervals from 7 to 10.5 ka (early Holocene), in the Younger Dryas (11.7-13 ka), from 15 to 17 ka (Heinrich event 1) and from 22.5 to 25 ka (Heinrich event 2) suggests completely different conditions of oxygen-rich bottom waters, extremely low mass and organic carbon accumulation rates, a high-diversity benthic fauna, all indicating lowered surface-water productivity. During these intervals the OMZ was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods. The abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the North Atlantic Heinrich events and dust peaks in the Tibetan Loess records. The correlation of the monsoon-driven OMZ variability in the Arabian Sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale.

Temperature reconstruction of sediment cores from the northeastern Arabian Sea

In order to reconstruct the monsoonal variability during the late Holocene we investigated a complete, annually laminated sediment record from the oxygen minimum zone (OMZ) off Pakistan for oxygen isotopes of planktic foraminifera and alkenone-derived sea surface temperatures (SST). Significant SST changes of up to 3°C which cannot be explained by changes in the alkenone-producing coccolithophorid species (inferred from the Gephyrocapsa oceanica / Emiliania huxleyi ratio) suggest that SST changes are driven by changes in the monsoon strength. Our high-(decadal)-resolution data indicate that the late Holocene in the northeastern Arabian Sea was not characterized by a stable uniform climate, as inferred from the Greenland ice cores, but by variations in the dominance of the SW monsoon conditions with significant effects on temperatures. Highest SST fluctuations of up to 3.0°C and 2.5°C were observed for the time interval from 4600 to 3300 years B.P. and during the past 500 years. The significant, short-term SST changes during the past 500 years might be related to climatic instabilities known from the northern latitudes ("Little Ice Age") and confirm global effects. Surface salinity values, reconstructed from delta18O records after correction for temperature-related oxygen isotope fractionation, suggest that in general, the past 5000 years were characterized by higher-than-recent evaporation and more intense SW monsoon conditions. However, between 4600 and 3700 years B.P., evaporation dropped, SW monsoon weakened, and NE monsoon conditions were comparatively enhanced. For the past 1500 years we infer strongly fluctuating monsoon conditions. Comparisons of reconstructed salinity records with ice accumulation data from published Tibetan ice core and Tibetan tree ring width data reveal that during the past 2000 years, enhanced evaporation in the northeastern Arabian Sea correlates with periods of increased ice accumulation in Tibet, and vice versa. This suggests a strong climatic relationship between both monsoon-controlled areas.

Carbonate mineralogical and isotopic analyses of TV-guided grab samples and TV-guided box corer samples from the Shumagin area

Methane seepage leads to Mg-calcite and aragonite precipitation at a depth of 4,850 m on the Aleutian accretionary margin. Stromatolitic and oncoid growth structures imply encrustation of microorganisms (microbial mats) in the host sediment with a unique growth direction downward into the sediment, forming crust-shaped lithologies. Biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol, which contain extremely low d13C values. This indicates the presence of methane-consuming archaea, and d13C values of -42 to -51 per mill PDB indicate that methane is the carbon source for the carbonate crusts. Thus, it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate-reducing bacteria and that carbonate precipitation proceeds downward into the sediment, where ascending cold fluids provide a methane source. Strontium and oxygen isotope analyses as well as 14C ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3,000 years ago.

Stable carbon and oxygen isotope composition of Late Cretaceous benthic foraminifera from the southern South Atlantic

The stable carbon and oxygen isotope composition of different benthic foraminiferal species of the latest Campanian and earliest Maastrichtian from Ocean Drilling Project Hole 690C (Weddell Sea, southern South Atlantic, ~1800 m paleowater depth) have been investigated. The total range of measured isotope values of all samples exceeds ~4 per mil for delta 13C and 1.1 per mil for delta 18O. Carbon isotope values of proposed deep infaunal species are generally similar or only slightly lower when compared to proposed epifaunal to shallow infaunal species. Interspecific differences vary between samples probably reflecting temporal changes in organic carbon fluxes to the sea floor. Constantly lower delta 13C values for Pullenia marssoni and Pullenia reussi suggest the deepest habitat for these species. The strong depletion of delta 13C values by up to 3 per mil within lenticulinids may be attributed to a deep infaunal microhabitat, strong vital effects, or different feeding strategy when compared to other species or modern lenticulinids. The mean delta 18O values reveal a strong separation of epifaunal to shallow infaunal and deep infaunal species. Epifaunal to shallow infaunal species are characterized by low delta 18O values, deep infaunal species by higher values. This result possibly reflects lower metabolic rates and longer life cycles of deep infaunal species or the operating of a pore water [CO3]2- effect on the benthic foraminiferal stable isotopes. Pyramidina szajnochae shows an enrichment of oxygen isotopes with test size comprising a total of 0.6 per mil between 250 and 1250 µm shell size. Although delta 13C lacks a corresponding trend these data likely represent the presence of changes in metabolic rates during ontogenesis. These results demonstrate the general applicability of multi-species stable isotope measurements of pristine Cretaceous benthic foraminifera to reconstruct past microhabitats and to evaluate biological and environmental effects on the stable isotope composition.

Age determinations and sedimentology on cores GIK17940 and 184-1144

Sedimentological, geochemical and paleomagnetic records were employed to reconstruct the history of East Asian Monsoon variability in the South China Sea (SCS) on orbital- and millennial-to-sub-decadal time scales. A detailed magnetostratigraphy for the southern central SCS was established as well as a stable isotope stratigraphy for ODP Site 1144 for the last 1.2 million years in the northern South China Sea. Furthermore a volcanic tephra layer from the southern central SCS could be identified as the Youngest Toba Ash, which thus re-presents an important age marker and was used to reconstruct paleo wind directions during the eruption 74 ka. Special attention was paid to the high- and ultrahigh-frequency variability in the last glacial-interglacial cycle and the Holocene, and to a precise age control of climate changes in general.

Sedimentology and stable oxygen isotope record of the northern South China Sea

High-resolution sediment records from the South China Sea reveal a winter monsoon dominated glacial regime and a summer monsoon dominated Holocene regime during the last glacial cycle. A fundamental change between regimes occurred during deglaciation through a series of millennial reoccurrences of century-scale changes in the East Asian monsoon (EAM) climate. These abrupt events centered at 17.0, 15.9, 15.5, 14.7, 13.5, 13.9, 13.3, 12.1, 11.5, and 10.7 14C ka correlate well with the millennial-scale events in the Santa Barbara Basin and the Arabian Sea, i.e. a relationship between EAM and El Niño/Southern Oscillation systems. The abrupt increases in summer monsoon imply enhanced heat transport from low-latitude sea area to the midlatitude/high-latitude land area. The phase relationship between events of EAM and ice sheet may reflect a faster EAM response and a slower ice sheet response to the insolation change. A far-reaching conclusion is that the EAM might have triggered the Northern Hemisphere deglaciation.

High-resolution multi-proxy record of the last interglacial period from Lake Van, eastern Anatolian high plateau, Turkey

A high-resolution multi-proxy record from Lake Van, eastern Anatolia, derived from a lacustrine sequence cored at the 357 m deep Ahlat Ridge (AR), allows a comprehensive view of paleoclimate and environmental history in the continental Near East during the last interglacial (LI). We combined paleovegetation (pollen), stable oxygen isotope (d18Obulk) and XRF data from the same sedimentary sequence, showing distinct variations during the period from 135 to 110 ka ago leading into and out of full interglacial conditions. The last interglacial plateau, as defined by the presence of thermophilous steppe-forest communities, lasted ca. 13.5 ka, from ~129.1-115.6 ka BP. The detailed palynological sequence at Lake Van documents a vegetation succession with several climatic phases: (I) the Pistacia zone (ca. 131.2-129.1 ka BP) indicates summer dryness and mild winter conditions during the initial warming, (II) the Quercus-Ulmus zone (ca. 129.1-127.2 ka BP) occurred during warm and humid climate conditions with enhanced evaporation, (III) the Carpinus zone (ca. 127.2-124.1 ka BP) suggest increasingly cooler and wetter conditions, and (IV) the expansion of Pinus at ~124.1 ka BP marks the onset of a colder/drier environment that extended into the interval of global ice growth. Pollen data suggest migration of thermophilous trees from refugial areas at the beginning of the last interglacial. Analogous to the current interglacial, the migration documents a time lag between the onset of climatic amelioration and the establishment of an oak steppe-forest, spanning 2.1 ka. Hence, the major difference between the last interglacial compared to the current interglacial (Holocene) is the abundance of Pinus as well as the decrease of deciduous broad-leaved trees, indicating higher continentality during the last interglacial. Finally, our results demonstrate intra-interglacial variability in the low mid-latitudes and suggest a close connection with the high-frequency climate variability recorded in Greenland ice cores.

(Table 1) Boron, carbon, magnesium and calcium isotope data, post-Marinoan Neoproterozoic carbonate sections, Namibia

A marked ocean acidification event and elevated atmospheric carbon dioxide concentrations following the extreme environmental conditions of the younger Cryogenian glaciation have been inferred from boron isotope measurements. Calcium and magnesium isotope analyses offer additional insights into the processes occurring during this time. Data from Neoproterozoic sections in Namibia indicate that following the end of glaciation the continental weathering flux transitioned from being of mixed carbonate and silicate character to a silicate-dominated one. Combined with the effects of primary dolomite formation in the cap dolostones, this caused the ocean to depart from a state of acidification and return to higher pH after climatic amelioration. Differences in the magnitude of stratigraphic isotopic changes across the continental margin of the southern Congo craton shelf point to local influences modifying and amplifying the global signal, which need to be considered in order to avoid overestimation of the worldwide chemical weathering flux.