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In a core off Pakistan, we obtained 38 14C analyses by accelerator mass spectrometry (AMS) from a 4.4-m-thick, expanded, annually-laminated Last Glacial Maximum (LGM) section, bracketed by bioturbated intervals ascribed to the Heinrich-1 (H1) and Heinrich-2 (H2) equivalent events (52 14C analyses between 24–15 kyr BP). A floating varve age scale, anchored to the oxygen isotope record of the layer-counted GISP2 ice core at the H2/LGM boundary, results in an annually dated record for the LGM from 23,450–17,900 cal BP. The floating varve scale of the LGM provides us with a tentative calibration of local marine AMS 14C age dates to calendar years.

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 δ18O 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.

A high-resolution laminated sediment record from the oxygen-minimum zone in the northeastern Arabian Sea off Pakistan was studied to reconstruct climatic variations. To infer the monsoon-driven “moisture history” in the northeastern Arabian Sea during the past 5000 years, we have used three types of independant proxies: (1) varve thickness, (2) stable isotope ratios of planktic foraminifera; and (3) inorganic geochemical composition. On the basis of these proxies we have subdivided the sedimentary record into five intervals. The first time interval from 5000 to 3900 yr BP (Period I) is dominated by summer-monsoon activity. Deduced from Ti/Al, Zr/Al, K/Al and Sr/Ca ratios and oxygen-isotope data of planktonic foraminifera slightly enhanced winter-monsoon conditions can be inferred for the period from 3900 to about 3000 yr BP (Period II). Maximum precipitation during enhanced winter-monsoon activity at about 3100 yr BP was followed by an onset of aridification at 3000 yr BP which continued until about 2000–2200 yr BP (Period III). The textural, mineralogical and geochemical data suggest that this aridification was a gradual, century-scale process rather than an abrupt event. Comparison to late Holocene records in NW- and SW-India, the Near East and tropical Africa supports our interpretation that Period III from 3000 to 2000 yr BP is characterized by a gradual change in the atmospherical moisture budget over much of this area. Reduced monsoonal rainfall may be linked to variations of Eurasian snow cover. As deduced from varve thickness and element to Al-ratios, Period IV from 2000 to about 1500 yr BP is characterized by a summer-monsoon dominated sequence with higher precipitation than during Period III. The δ18O record shows an abrupt shift at 2000 yr BP coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern Arabian Sea. Present-day conditions in the Makran area were established at 1500 yr BP (Period V). Based on inorganic geochemical data and meteorological records at several stations in the Makran we conclude that Period V is dominated by precipitation during the winter season. Here, declined rainfall associated with the summer-monsoon may be related to a less northward position of the intertropical convergence zone (ITCZ) during summer and a stronger influence of winter-monsoon linked phenomenona.

The Makran accretionary complex shows a distinct bottom-simulating reflector, indicating a thick gas-hydrate-bearing horizon between the deformational front and about 1350 m water depth which seals off the upward flow of gas-charged fluids. A field of presently inactive mud diapirs with elevations up to 65 m was discovered in the abyssal plain seawards of the deformation front, suggesting that in the past conditions were favorable for periodic but localized vigorous mud diapirism. Regional destabilization of the gas hydrate leading to focused flow was observed where deep-penetrating, active faults reach the base of the gas-hydrate layer, as in a deeply incised submarine canyon (2100–2500 m water depth). At this location we discovered seeps of methane and H2S-rich fluids associated with chemoautotrophic vent faunas (e.g., Calyptogena sp.). Driven by the accretionary wedge dynamics, the landward part of the gas-hydrate layer below the Makran margin is being progressively uplifted. Due to reduced hydrostatic pressure and rising ocean bottom-water temperatures, gas hydrates are progressively destabilized and dissociated into hydrate water, methane and H2S. Sediment temperatures lie outside the methane stability field wherever water depth is less than 800 m. Above this depth, upward migration of fluids to the seafloor is unimpeded, thus explaining the abundance of randomly distributed gas seeps observed at water depths of 350 to 800 m.

Sea surface temperature (SST) estimates were made using data for C37-39 alkenones analyzed in modern and glacial-age intervals of sediment cores collected along an east-west (∼42°N from 125° to 132°W) and a north-south (41°–33°N) transect of the California Current system (CCS). The prymnesiophyte biomarker results suggest that surface waters warmed significantly throughout this region since the last glacial maximum (LGM) but the magnitude of warming varied spatially. Stratigraphic profiles from four sediment cores along the north-south transect indicate the warming period was confined to the glacial/interglacial transition (15-10 ka), with SST reaching a maximum value at ∼10 ka and maintaining a uniformly high value throughout the Holocene. Comparison of estimates derived from analysis of modern and LGM sediment intervals indicates the magnitude of the SST change was minimal for locations south of about 36°N (≤1°–2°C) and increased significantly (3°–5°C) north of this latitude. Using a simple heat balance model, we calculate from the latitudinal gradient in SST that southward flow in the California Current system during the LGM was about 60% of that measured today at 36°N. Our findings support the conclusion of others based on pollen data that coastal upwelling in the region of the northeast Pacific off northern California was significantly reduced or even completely shutdown during the LGM.

Arabian Sea sediments record changes in the upwelling system off Arabia, which is driven by the monsoon circulation system over the NW Indian Ocean. In accordance with climate models, and differing from other large upwelling areas of the tropical ocean, a 500,000-yr record of productivity at ODP Site 723 shows consistently stronger upwelling during interglaciations than during glaciations. Sea-surface temperatures (SSTs) reconstructed from the alkenone unsaturation index (UK′37) are high (up to 27°C) during interglaciations and low (22-24°C) during glaciations, indicating a glacial-interglacial temperature change of >3°C in spite of the dampening effect of enhanced or weakened upwelling. The increased productivity is attributed to stronger monsoon winds during interglacial times relative to glacial times, whereas the difference in SSTs must be unrelated to upwelling and to the summer monsoon intensity. The winter (NE) monsoon was more effective in cooling the Arabian Sea during glaciations then it is now.