Sea surafe temperture calculation for the Arabian Sea

Sea surface temperature (SST) and seawater d18O (d18Ow) were reconstructed in a suite of sediment cores from throughout the Arabian Sea for four distinct time intervals (0 ka, 8 ka, 15 ka, and 20 ka) with the aim of understanding the history of the Indian Monsoon and the climate of the Arabian Sea region. This was accomplished through the use of paired Mg/Ca and d18O measurements of the planktonic foraminifer Globigerinoides ruber. By analyzing basin-wide changes and changes in cross-basinal gradients, we assess both monsoonal and regional-scale climate changes. SST was colder than present for the majority of sites within all three paleotime slices. Furthermore, both the Indian Monsoon and the regional Arabian Sea mean climate have varied substantially over the past 20 kyr. The 20 ka and 15 ka time slices exhibit average negative temperature anomalies of 2.5°-3.5°C attributable, in part, to the influences of glacial atmospheric CO2 concentrations and large continental ice sheets. The elimination of the cross-basinal SST gradient during these two time slices likely reflects a decrease in summer monsoon and an increase in winter monsoon strength. Changes in d18Ow that are smaller than the d18O signal due to global ice volume reflect decreased evaporation and increased winter monsoon mixing. SSTs throughout the Arabian Sea were still cooler than present by an average of 1.4°C in the 8 ka time slice. These cool SSTs, along with lower d18Ow throughout the basin, are attributed to stronger than modern summer and winter monsoons and increased runoff and precipitation. The results of this study underscore the importance of taking a spatial approach to the reconstruction of processes such as monsoon upwelling.

Stable oxygen isotope record and lithogenic composition of ODP Hole 117-722B

The upper 38 m of Hole 722B sediments (Owen Ridge, northwest Arabian Sea) was sampled at 20 cm intervals and used to develop records of lithogenic percent, mass accumulation rate, and grain size spanning the past 1 m.y. Over this interval, the lithogenic component of Owen Ridge sediments can be used to infer variability in the strength of Arabian Sea summer monsoon winds (median grain size) and the aridity of surrounding dust source-areas (mass accumulation rate; MAR in g/cm**2/k.y). The lithogenic MAR has strong 100, 41, and 23 k.y. cyclicities and is forced primarily by changes in source-area aridity associated with glacial-interglacial cycles. The lithogenic grain size, on the other hand, exhibits higher frequency variability (23 k.y.) and is forced by the strength of summer monsoon winds which, in turn, are forced by the effective sensible heating of the Indian-Asian landmass and by the availability of latent heat from the Southern Hemisphere Indian Ocean. These forcing mechanisms combine to produce a wind-strength record which has no strong relationship to glacial-interglacial cycles. Discussion of the mechanisms responsible for production of primary Milankovitch cyclicities in lithogenic records from the Owen Ridge is presented elsewhere (Clemens and Prell, 1990, doi:10.1029/PA005i002p00109). Here we examine the 1 m.y. record from Hole 722B focusing on different aspects of the lithogenic components including an abrupt change in the monsoon wind-strength record at 500 k.y., core-to-core reproducibility, comparison with magnetic susceptibility, coherency with a wind-strength record from the Pacific Ocean, and combination frequencies in the wind-strength record. The Hole 722B lithogenic grain-size record shows an abrupt change at 500 k.y. possibly indicating decreased monsoon wind-strength over the interval from 500 k.y. to present. The grain-size decrease appears to be coincident with a loss of spectral power near the 41 k.y. periodicity. However, the grain-size decrease is not paralleled in the Globigerina bulloides upwelling record, an independent record of summer monsoon wind-strength (Prell, this volume). These observations leave us with competing hypotheses possibly involving: (1) a decrease in the sensitivity of monsoon windstrength to obliquity forcing, (2) decoupling of the grain size and G. bulloides records via a decoupling of the nutrient supply from wind-driven upwelling, and/or (3) a change in dust source-area or the patterns of dust transporting winds. Comparison of the lithogenic grain size and weight percent records from Hole 722B with those from a nearby core shows that the major and most minor events are well replicated. These close matches establish our confidence in the lithogenic extraction techniques and measurements. Further, reproducibility on a core-to-core scale indicates that the eolian depositional signal is regionally strong, coherent, and well preserved. The lithogenic weight percent and magnetic susceptibility are extremely well correlated in both the time and frequency domains. From this we infer that the magnetically susceptible component of Owen Ridge sediments is of terrestrial origin and transported to the Owen Ridge via summer monsoon winds. Because of the high correlation with the lithogenic percent record, the magnetic susceptibility record can be cast in terms of lithogenic MAR and used as a high resolution proxy for continental aridity. In addition to primary Milankovitch periodicities, the Hole 722B grain-size record exhibits periodicity at 52 k.y. and at 29 k.y. Both periodicities are also found in the grain-size record from piston core RC11-210 in the equatorial Pacific Ocean. Comparison of the two grain-size records shows significant coherence and zero phase relationships over both the 52 and 29 k.y. periodicities suggesting that the strengths of the Indian Ocean monsoon and the Pacific southeasterly trade winds share common forcing mechanisms. Two possible origins for the 52 and 29 k.y. periodicities in the Hole 722B wind-strength record are (1) direct Milankovitch forcing (54 and 29 k.y. components of obliquity) and (2) combination periodicities resulting from nonlinear interactions within the climate system. We find that the 52 and 29 k.y. periodicities show stronger coherency with crossproducts of eccentricity and obliquity (29 k.y.) and precession and obliquity (52 k.y.) than with direct obliquity forcing. Our working hypothesis attributes these periodicities to nonlinear interaction between external insolation forcing and internal climatic feedback mechanisms involving an interdependence of continental snow/ice-mass (albedo) and the hydrological cycle (latent heat availability).

Chronology and stable carbon isotopic ratios of sediments from the Western Pacific Warm Pool region

Instrumental data suggest that major shifts in tropical Pacific atmospheric dynamics and hydrology have occurred within the past century, potentially in response to anthropogenic warming. To better understand these trends, we use the hydrogen isotopic ratios of terrestrial higher plant leaf waxes (DDwax) in marine sediments from southwest Sulawesi, Indonesia, to compile a detailed reconstruction of central Indo-Pacific Warm Pool (IPWP) hydrologic variability spanning most of the last two millennia. Our paleodata are highly correlated with a monsoon reconstruction from Southeast Asia, indicating that intervals of strong East Asian summer monsoon (EASM) activity are associated with a weaker Indonesian monsoon (IM). Furthermore, the centennial-scale oscillations in our data follow known changes in Northern Hemisphere climate (e.g., the Little Ice Age and Medieval Warm Period) implying a dynamic link between Northern Hemisphere temperatures and IPWP hydrology. The inverse relationship between the EASM and IM suggests that migrations of the Intertropical Convergence Zone and associated changes in monsoon strength caused synoptic hydrologic shifts in the IPWP throughout most of the past two millennia.

Oxygen isotope and Mg/Ca record of planktonic foraminifera of sediment core MD01-2390

Changes in the local freshwater budget over the last 22,000 years have been estimated from a sediment core located in the southern South China Sea (SCS) using a combined approach of Mg/Ca and oxygen isotopes on the planktonic foraminifera Globigerinoides ruber (white) sensu stricto (s.s.). Core MD01-2390 (06°28,12N, 113°24,56E; water depth 1591 m) is located near the glacial paleo-river mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers that drained the exposed Sunda Shelf. The delta18Oseawater record reveals lower average values (-0.96±0.18 per mil) during the Last Glacial Maximum (LGM) when compared with modern values (-0.54±0.18 per mil). Low salinity during the LGM is interpreted to reflect a higher freshwater contribution due to a greater proximity of the core site to the mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers at that time. A general deglacial increasing trend in salinity due to the progressive landward displacement of the coastline during deglacial shelf flooding is punctuated by several short-term shifts towards higher and lower salinity that are likely related to abrupt changes in the intensity of the East Asian summer monsoon. Thus, the deglacial delta18Oseawater changes reflect the combined effects of sea-level-induced environmental changes on the shelf (e.g. phases of retreat and breakdown of the shelf drainage systems) and East Asian monsoon climate change. Lower salinity than at present during the Early Holocene may be attributed to an increase in summer monsoonal precipitation that is corroborated by previous marine and terrestrial studies that report a Preboreal-Early Holocene monsoon optimum in the Asian monsoon region.

Pollen record and calculated holocene vegetation and climate dynamics from Sihailongwan Maar Lake, NE China

High-resolution palynological analysis on annually laminated sediments of Sihailongwan Maar Lake (SHL) provides new insights into the Holocene vegetation and climate dynamics of NE China. The robust chronology of the presented record is based on varve counting and AMS radiocarbon dates from terrestrial plant macro-remains. In addition to the qualitative interpretation of the pollen data, we provide quantitative reconstructions of vegetation and climate based on the method of biomization and weighted averaging partial least squares regression (WA-PLS) technique, respectively. Power spectra were computed to investigate the frequency domain distribution of proxy signals and potential natural periodicities. Pollen assemblages, pollen-derived biome scores and climate variables as well as the cyclicity pattern indicate that NE China experienced significant changes in temperature and moisture conditions during the Holocene. Within the earliest phase of the Holocene, a large-scale reorganization of vegetation occurred, reflecting the reconstructed shift towards higher temperatures and precipitation values and the initial Holocene strengthening and northward expansion of the East Asian summer monsoon (EASM). Afterwards, summer temperatures remain at a high level, whereas the reconstructed precipitation shows an increasing trend until approximately 4000 cal. yr BP. Since 3500 cal. yr BP, temperature and precipitation values decline, indicating moderate cooling and weakening of the EASM. A distinct periodicity of 550-600 years and evidence of a Mid-Holocene transition from a temperature-triggered to a predominantly moisture-triggered climate regime are derived from the power spectra analysis. The results obtained from SHL are largely consistent with other palaeoenvironmental records from NE China, substantiating the regional nature of the reconstructed vegetation and climate patterns. However, the reconstructed climate changes contrast with the moisture evolution recorded in S China and the mid-latitude (semi-)arid regions of N China. Whereas a clear insolation-related trend of monsoon intensity over the Holocene is lacking from the SHL record, variations in the coupled atmosphere-Pacific Ocean system can largely explain the reconstructed changes in NE China.

Holocene corals from the South China Sea

The South China Sea (SCS) is well connected with the western Pacific and influenced by the East Asian monsoon. We have examined temporal variations in radiocarbon marine reservoir ages (R) and regional marine reservoir corrections (DeltaR) of the SCS during the Holocene using paired measurements of AMS 14C and TIMS 230Th on 20 pristine corals. The results show large fluctuations in both R and DeltaR values over the past 7500 years (yrs) with two distinct plateaus during 7.5-5.6 and 3.5-2.5 thousand calendar years before present (cal ka BP). The respective weighted mean DeltaR values of these plateaus are 151 ± 85 and 89 ± 59 yrs, which are significantly higher than its modern value of -23 ± 52 yrs. This suggests that using a constant modern DeltaR value to calibrate 14C dates of the SCS marine samples will introduce additional errors to the calibrated ages. Our results provide the first database for the Holocene R and DeltaR values of the SCS for improved radiocarbon calibration of marine samples. We interpret the two DeltaR plateaus as being related to two intervals with weakened El Niño - Southern Oscillation (ENSO) and intensified East Asian summer monsoon (EASM). This is because the 14C content of the SCS surface water is controlled by both the 14C concentration of the Pacific North Equatorial Current (NEC) which is in turn influenced by ENSO-induced upwelling along the Pacific equator and vertical upwelling within the SCS as a result of moisture transportation to midlatitude region to supply the EASM rainfall.

Biogeochemical investigations on sediment core MD04-2876

Modern seawater profiles of oxygen, nitrate deficit, and nitrogen isotopes reveal the spatial decoupling of summer monsoon-related productivity and denitrification maxima in the Arabian Sea (AS) and raise the possibility that winter monsoon and/or ventilation play a crucial role in modulating denitrification in the northeastern AS, both today and through the past. A new high-resolution 50-ka record of d15N from the Pakistan margin is compared to five other denitrification records distributed across the AS. This regional comparison unveils the persistence of east-west heterogeneities in denitrification intensity across millennial-scale climate shifts and throughout the Holocene. The oxygen minimum zone (OMZ) experienced east-west swings across Termination I and throughout the Holocene. Probable causes are (1) changes in ventilation due to millennial-scale variations in Antarctic Intermediate Water formation and (2) postglacial reorganization of intermediate circulation in the northeastern AS following sea level rise. Whereas denitrification in the world's OMZs, including the western AS, gradually declined following the deglacial maximum (10-9 ka BP), the northeastern AS record clearly witnesses increasing denitrification from about 8 ka BP. This would have impacted the global Holocene climate through sustained N2O production and marine nitrogen loss.

Elemental analyses (XRF), total organic carbon (TOC), grain size analyses, lamination index of sediment core SO130-289KL

The Dansgaard-Oeschger oscillations and Heinrich events described in North Atlantic sediments and Greenland ice are expressed in the climate of the tropics, for example, as documented in Arabian Sea sediments. Given the strength of this teleconnection, we seek to reconstruct its range of environmental impacts. We present geochemical and sedimentological data from core SO130-289KL from the Indus submarine slope spanning the last ~ 80 kyr. Elemental and grain size analyses consistently indicate that interstadials are characterized by an increased contribution of fluvial suspension from the Indus River. In contrast, stadials are characterized by an increased contribution of aeolian dust from the Arabian Peninsula. Decadal-scale shifts at climate transitions, such as onsets of interstadials, were coeval with changes in productivity-related proxies. Heinrich events stand out as especially dry and dusty events, indicating a dramatically weakened Indian summer monsoon, potentially increased winter monsoon circulation, and increased aridity on the Arabian Peninsula. This finding is consistent with other paleoclimate evidence for continental aridity in the northern tropics during these events. Our results strengthen the evidence that circum-North Atlantic temperature variations translate to hydrological shifts in the tropics, with major impacts on regional environmental conditions such as rainfall, river discharge, aeolian dust transport, and ocean margin anoxia.

Pollen ages and depths in ODP Site 184-1144

A high-resolution pollen record (sampling interval averages 820 years) has been obtained from ODP Site 1144 (water depth 2037 m), northern South China Sea. The 504-m sequence (in composition length) covers the last 1.03 million years according to micropaleontological and isotopic stratigraphy. The pollen assemblages are characterized by high proportions of Pinus and herb pollen, and by their frequent alternations. Based on these alternations, 29 pollen zones have been recognized that are closely correlated to the Marine Oxygen Isotope Stages (MIS) 1-29. Pinus- dominant pollen zones correspond to interglacial periods with lighter delta18O values, while herb-marked ones relate to the heavier delta18O stages assigned to glacials. Judging from the pollen data, the exposed northern continental shelf of the South China Sea during the glacials was covered by grassland, and the extensive northern shelf has formed only since MIS 6 (ca. 150 ka), probably as a result of tectonic subsidence. Tree pollen influx values are indicative of winter monsoon which began to intensify 600 ka ago. The summer monsoon variations can be approximated by the fern percentage within the total pollen and spore abundance, and the result shows high values in general occurring at interglacials, with the maxima at MIS 15, 5e and 1. The relatively high fern percentage with smaller amplitude in variations before 600 ka might suggest more stable humid conditions before the intensification of winter monsoon.

Organic carbon content, stable isotopes and planktonic foraminifera in sediments of tghe norther Arabian Sea oxygen minimum zone

The northern Arabian Sea is one of the few regions in the open ocean where thermocline water is severely depleted in oxygen. The intensity of this oxygen minimum zone (OMZ) has been reconstructed over the past 225,000 years using proxies for surface water productivity, water column denitrification, winter mixing, and the aragonite compensation depth (ACD). Changes in OMZ intensity occurred on orbital and suborbital timescales. Lowest O2 levels correlate with productivity maxima and shallow winter mixing. Precession-related productivity maxima lag early summer insolation maxima by ~6 kyr, which we attribute to a prolonged summer monsoon season related to higher insolation at the end of the summer. Periods with a weakened or even non-existent OMZ are characterized by low productivity conditions and deep winter mixing attributed to strong and cold winter monsoonal winds. The timing of deep winter mixing events corresponds with that of periods of climatic cooling in the North Atlantic region.

Median grain size in ODP Sites 184-1143 and 184-1144

Monsoon climate is an important component of the global climatic system. A comprehensive understanding of its variability over glacial-interglacial time scales as well as of its effects on the continent and in the ocean is required to decipher links between climate, continental weathering and productivity. A detailed multiproxy study, including bulk and clay mineralogy, grain-size analysis, phosphorus geochemistry (SEDEX extraction), organic matter characterization, and nitrogen stable isotopes, was carried out on samples from ODP Sites 1143 and 1144 (Leg 184, South China Sea), covering the past 140 000 years. We tentatively reconstruct the complex sedimentation and climatic history of the region during the last glacial-interglacial cycle, when sea-level variations, linked to the growth and melting of ice caps, interact with monsoon variability. During interglacial periods of high sea level, summer monsoon was strong, and humid and warm climate characterized the adjacent continent and islands. Clay minerals bear signals of chemical weathering during these intervals. High calcite and reactive phosphorus mass accumulation rates (MARs) indicate high productivity, especially in the southern region of the basin. During glacial intervals, strong winter monsoon provided enhanced detrital input from the continent, as indicated by high detrital MAR. Glacial low sea level resulted in erosion of sediments from the exposed Sunda shelf to the south, and clay mineral variations indicate that warm and humid conditions still prevailed in the southern tropical areas. Enhanced supply of nutrients from the continent, both by river and eolian input, maintained high primary productivity. Reduced circulation during these periods possibly induced active remobilization of nutrients, such as phosphorus, from the sediments. Intense and short cold periods recorded during glacial and interglacial stages correlate with loess records in China and marine climatic records in the North Atlantic, confirming a teleconnection between low- and high-latitude climate variability.

Radiocarbon datings and temperature reconstructions of sediment core GeoB10038-4 off SW Sumatra

Surface and thermocline conditions of the eastern tropical Indian Ocean were reconstructed through the past glacial-interglacial cycle by using Mg/Ca and alkenone-paleothermometry, stable oxygen isotopes of calcite and seawater, and terrigenous fraction performed on sediment core GeoB 10038-4 off SW Sumatra (~6°S, 103°E, 1819 m water depth). Results show that annual mean surface and thermocline temperatures varied differently and independently, and suggest that surface temperatures have been responding to southern high-latitude climate, whereas the more variable thermocline temperatures were remotely controlled by changes in the thermocline temperatures of the North Indian Ocean. Except for glacial terminations, salinity proxies indicate that changing intensities of the boreal summer monsoon did not considerably affect annual mean conditions off Sumatra during the past 133,000 years. Our results do not show a glacial-interglacial pattern in the thermocline conditions and reject a linear response of the tropical Indian Ocean thermocline to mid- and high-latitude climate change. Alkenone-based surface temperature estimates varied in line with the terrigenous fraction of the sediment and the East Asian winter monsoon proxy records at the precession band suggestive of monsoon (sea level) to be the dominant control on alkenone temperatures in the eastern tropical Indian Ocean on sub-orbital (glacial-interglacial) timescales.

(Table 1) Stable carbon isotope record and total nitrogen concentration in ODP Hole 175-1085B sediments

Variations in the strength of coastal upwelling in the South East Atlantic Ocean and summer monsoonal rains over South Africa are controlled by the regional atmospheric circulation regime. Although information about these parameters exists for the last glacial period, little detailed information exists for older time periods. New information from ODP Site 1085 for Marine Isotope Stages (MIS) 12-10 shows that glacial-interglacial productivity trends linked to upwelling variability followed a pattern similar to the last glacial cycle, with maximums shortly before glacial maxima, and minimums shortly before glacial terminations. During the MIS-11/10 transition, several periodic oscillations in productivity and monsoonal proxies are best explained by southwards shifts in the southern sub-tropical high-pressure cells followed by abrupt northwards shifts. Comparison to coeval sea-surface temperature measurements suggests that these monsoonal cycles were tightly coupled to anti-phased hemispheric climate change, with an intensified summer monsoon during periods of Northern (Southern) Hemisphere cooling (warming). The timing of these events suggests a pacing by insolation over precession periods. A lack of similar regional circulation shifts during the MIS-13/12 transition is likely due to the large equatorwards shift in the tropical convection zone that occurred during this extreme glaciation.

Fossil pollen record of composite sediment core TMD from Tso Moriri, analysis of modern surface pollen samples, mean annual precipitation reconstruction, and digitisation and recalibration of different discussed palaeoclimate proxy records

This paper presents a new fossil pollen record from Tso Moriri (32°54'N, 78°19'E, 4512 m a.s.l.) and seeks to reconstruct changes in mean annual precipitation (MAP) during the last 12,000 years. This high-alpine lake occupies an area of 140 km**2 in a glacial-tectonic valley in the northwestern Himalaya. The region has a cold climate, with a MAP <300 mm, and open vegetation. The hydrology is controlled by the Indian Summer Monsoon (ISM), but winter westerly-associated precipitation also affects the regional water balance. Results indicate that precipitation levels varied significantly during the Holocene. After a rapid increase in MAP, a phase of maximum humidity was reached between ca. 11 to 9.6 cal ka BP, followed by a gradual decline in MAP. This trend parallels the reduction in the Northern Hemisphere summer insolation. Comparison of different palaeoclimate proxy records reveal evidence for a stronger Holocene decrease in precipitation in the northern versus the southern parts of the ISM domain. The long-term trend of ISM weakening is overlaid with several short periods of greater dryness, which are broadly synchronous with the North Atlantic cold spells, suggesting reduced amounts of westerly-associated winter precipitation. Compared to the mid and late Holocene, it appears that westerlies had a greater influence on the western parts of the ISM domain during the early Holocene. During this period, the westerly-associated summer precipitation belt was positioned at Mediterranean latitudes and amplified the ISM-derived precipitation. The Tso Moriri pollen record and moisture reconstructions also suggest that changes in climatic conditions affected the ancient Harappan Civilisation, which flourished in the greater Indus Valley from approximately 5.2 to 3 cal ka BP. The prolonged Holocene trend towards aridity, punctuated by an interval of increased dryness (between ca. 4.5 to 4.3 cal ka BP), may have pushed the Mature Harappan urban settlements (between ca. 4.5 to 3.9 cal ka BP) to develop more efficient agricultural practices to deal with the increasingly acute water shortages. The amplified aridity associated with North Atlantic cooling between ca. 4 to 3.6 and around 3.2 cal ka BP further hindered local agriculture, possibly causing the deurbanisation that occurred from ca. 3.9 cal ka BP and eventual collapse of the Harappan Civilisation between ca. 3.5 to 3 cal ka BP.

Uranium-Thorium dating and sea surface temperatures from corals from Hainan Island, South China Sea

Three mid-Holocene sea surface temperature (SST) records spanning more than 30 years were reconstructed for the northern South China Sea using Sr/Ca ratios in Porites corals. The results indicate warmer than present climates between circa 6100 yr B.P. and circa 6500 yr B.P. with the mid-Holocene average minimum monthly winter SSTs, the average maximum monthly summer SSTs, and the average annual SSTs being about 0.5°-1.4°C, 0°-2.0°C, and 0.2°-1.5°C higher, respectively, than they were during 1970-1994. Summer SSTs decrease from circa 6500 yr B.P. to circa 6100 yr B.P. with a minimum centered at circa 6300 yr B.P. The higher average summer SSTs are consistent with a stronger summer monsoon during the mid-Holocene, and the decreasing trend indicates a secular decrease of summer monsoon strength, which reflects the change in summer insolation in the Northern Hemisphere. El Niño-Southern Oscillation (ENSO) cycles were apparent in both the mid-Holocene coral and modern instrumental records. However, the ENSO variability in the mid-Holocene SSTs was weaker than that in the modern record, and the SST record with the highest summer temperatures from circa 6460 yr B.P. to 6496 yr B.P. shows no robust ENSO cycle. This agrees with other studies that indicate that stronger summer monsoon circulation may have been associated with suppressed ENSO variability during the mid-Holocene.