Search

During the middle Miocene, Earth's climate changed from a global warm period (Miocene Climatic Optimum) into a colder one with the expansion of the Antarctic ice sheet. This prominent climate transition was also a period of drastic changes in global atmospheric circulation. The development of the South Asian monsoon is not well understood and mainly derived from proxy records of wind strength. Data for middle Miocene changes in rainfall are virtually non‐existent for India and the Arabian Sea prior to 11 Ma. This study presents planktic foraminiferal trace element (Mg/Ca and Ba/Ca) and stable oxygen isotope records from NGHP‐01 Site 01A off the coast of West India in the Eastern Arabian Sea (EAS) to reconstruct the regional surface hydrography and hydroclimate in the South Asian monsoon (SAM) region during the middle Miocene. The Ba/Ca and local seawater δ18O (δ18Osw) changes reveal a notable gradual increase in SAM rainfall intensity during the middle Miocene. Additionally to this long‐term increase in precipitation, the seawater δ18O is punctuated by a prominent decrease, i.e. freshening, at ~14 Ma contemporary with Antarctic glaciation. This suggests that Southern Ocean Intermediate Waters (SOIW) transmitted Antarctic salinity changes into the Arabian Sea via an "oceanic tunnel" mechanism. The middle Miocene increase in SAM rainfall is consistent with climate model simulations of an overall strengthening Asian monsoon from the Eocene to the middle/late Miocene with a further acceleration after the middle Miocene climate transition. Key Points: We present the first South Asian monsoon (SAM) rainfall record for the Eastern Arabian Sea region for the middle to late MioceneWe find an intensification of SAM rainfall from ~15 to 11 MaIntensification of the SAM is consistent with climate models of an overall strengthening Asian monsoon from the Eocene to late Miocene [ABSTRACT FROM AUTHOR]

The response of the tropical climate in the Indian Ocean realm to abrupt climate change events in the North Atlantic Ocean is contentious. Repositioning of the intertropical convergence zone is thought to have been responsible for changes in tropical hydroclimate during North Atlantic cold spells, but the dearth of high-resolution records outside the monsoon realm in the Indian Ocean precludes a full understanding of this remote relationship and its underlying mechanisms. Here we show that slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials and the Younger Dryas stadial affected the tropical Indian Ocean hydroclimate through changes to the Hadley circulation including a southward shift in the rising branch (the intertropical convergence zone) and an overall weakening over the southern Indian Ocean. Our results are based on new, high-resolution sea surface temperature and seawater oxygen isotope records of well-dated sedimentary archives from the tropical eastern Indian Ocean for the past 45,000 years, combined with climate model simulations of Atlantic circulation slowdown under Marine Isotope Stages 2 and 3 boundary conditions. Similar conditions in the east and west of the basin rule out a zonal dipole structure as the dominant forcing of the tropical Indian Ocean hydroclimate of millennial-scale events. Results from our simulations and proxy data suggest dry conditions in the northern Indian Ocean realm and wet and warm conditions in the southern realm during North Atlantic cold spells. [ABSTRACT FROM AUTHOR]

Abstract: Millennial-scale variations of the East Asian winter monsoon (EAWM) remain elusive due to sparse and controversial reconstructions. By compiling a variety of alkenone-based sea surface temperature (SST) estimates, we find that the west–east SST gradient in the southern South China Sea (SCS) well documents the temporal dynamics of the winter “cold tongue” off the southern Vietnam and by inference, variations in the EAWM intensity over the past 26ka. Our results reveal that the winter “cold tongue” SSTs were significantly colder during Heinrich event 1 and the Younger Dryas event, resulting in an increased west–east SST gradient in the southern SCS due to a strengthened EAWM. Within dating uncertainties, an intensified EAWM during cold stadials was coeval with the shutdown or a reduction in strength of the Atlantic meridional overturning circulation (AMOC), exhibiting a strong linkage between the AMOC and the EAWM system. The west–east SST gradient also indicates an enhanced EAWM during the early Holocene, which may be induced by postglacial ice-sheet dynamics and a strong seasonal contrast in solar insolation. Our findings suggest that the EAWM was probably modulated by a complex interplay between the AMOC, solar insolation and ice-sheet dynamics on sub-orbital time scales. [ABSTRACT FROM AUTHOR]

Abstract: The monsoon system represents one of the basic elements of global atmospheric circulation. Its evolution and variability over long periods of geologic time play a significant role in our understanding of global climate. In this study, we focus on the Late Miocene interval from 10Ma to 6Ma, a period of postulated profound ecological and environmental shifts in East and South Asia. The combined approach of measuring planktonic foraminiferal Mg/Ca ratios and stable oxygen isotopes from Ocean Drilling Program (ODP) Site 1146 enabled us to reconstruct temperature independent seawater δ 18O (i.e. proxy for sea surface salinity) variations in order to reconstruct the hydrography in the northern South China Sea. Located offshore the Pearl (Zhujiang) River, or its predecessor, the location of ODP Site 1146 is considered to provide a most sensitive record for detecting potential changes in freshwater input/river run-off as a result of changes in continental humidity, and hence changes in East Asian summer monsoon (EASM) climate. Local seawater δ 18O reconstructions reveal that monsoon development during the Late Miocene can be regarded as an abrupt weakening in the EASM around 7.5Ma. We suggest that an EASM weakening was most likely the driving force for decreasing aridity in East and South Asia at 8–6Ma, leading to widespread ecosystem changes in East and South Asia. [Copyright &y& Elsevier]

Abstract: Quantifying the spatial and temporal sea surface temperature (SST) and salinity changes of the Indo-Pacific Warm Pool is essential to understand the role of this region in connection with abrupt climate changes particularly during the last deglaciation. In this study we reconstruct SST and seawater δ 18O of the tropical eastern Indian Ocean for the past 40,000years from two sediment cores (GeoB 10029-4, 1°30′S, 100°08′E, and GeoB 10038-4, 5°56′S, 103°15′E) retrieved offshore Sumatra. Our results show that annual mean SSTs increased about 2–3°C at 19,000years ago and exhibited southern hemisphere-like timing and pattern during the last deglaciation. Our SST records together with other Mg/Ca-based SST reconstructions around Indonesia do not track the monsoon variation since the last glacial period, as recorded by terrestrial monsoon archives. However, the spatial SST heterogeneity might be a result of changing monsoon intensity that shifts either the annual mean SSTs or the seasonality of G. ruber towards the warmer or the cooler season at different locations. Seawater δ 18O reconstructions north of the equator suggest fresher surface conditions during the last glacial and track the northern high-latitude climate change during the last deglaciation. In contrast, seawater δ 18O records south of the equator do not show a significant difference between the last glacial period and the Holocene, and lack Bølling-Allerød and Younger Dryas periods suggestive of additional controls on annual mean surface hydrology in this part of the Indo-Pacific Warm Pool. [Copyright &y& Elsevier]

Abstract: Sea surface temperatures (SSTs), reconstructed from two Globigerinoides ruber (white) morphotypes (G. ruber sensu stricto, (s.s.); G. ruber sensu lato, ( s.l.)) Mg/Ca and alkenones (UK′ 37) from core MD01-2390 from the tropical South China Sea (SCS) during the last deglaciation reveal a proxy-dependent discrepancy in the temporal pattern of the deglacial warming. Alkenone data suggest that the deglacial warming is punctuated by a decrease in temperature between ∼17 and 15ka BP, corroborating previously published alkenone UK′ 37 SST records from the southern SCS. Within dating uncertainties, this cooling is coeval with the Heinrich Event 1 (H1) time interval in the North Atlantic region, underscoring the imprints of northern hemisphere forcing on tropical SCS ocean temperatures. The deglacial UK′ 37 SST minimum is also paralleled by a maximum in G. ruber morphotype-specific δ18O. G. ruber Mg/Ca SST estimates suggest a morphotype-specific record of SSTs during the time interval of H1. Whereas G. ruber s.s. imply a continuous warming starting around 18ka BP without any marked response to H1, G. ruber s.l.-based Mg/Ca SST estimates reveal a cooling around ∼17–15ka BP similar to the H1 interval cooling seen in the alkenone SST record. Similar proxy-dependent differences in deglacial surface water warming have been recorded in the eastern equatorial Pacific, implying a common pattern on both sides of the tropical Pacific Ocean. We submit that this discrepancy could be due to differences in seasonality of planktonic foraminifera G. ruber morphotypes and alkenone-producing algae. [Copyright &y& Elsevier]

Abstract: 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 1591m) is located near the glacial paleo-river mouths of the Baram, Rajang and North Sunda/Molengraaff Rivers that drained the exposed Sunda Shelf. The δ 18Oseawater record reveals lower average values (−0.96±0.18‰) during the Last Glacial Maximum (LGM) when compared with modern values (−0.54±0.18‰). 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 δ 18Oseawater 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. [Copyright &y& Elsevier]

Abstract: 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, 1819m 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 133,000years. 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. [Copyright &y& Elsevier]

El Niño/Southern Oscillation (ENSO) events and latitudinal shifts of the Intertropical Convergence Zone (ITCZ) cause large scale hydrologic changes in the western tropical Pacific (WTP). However, there is still a debate on the role of ENSO and ITCZ shifts on the hydroclimate in the WTP and their linkage to Northern Hemisphere climate change on glacial-interglacial timescales. For that reason, we present ice volume-corrected seawater δ 18 O (δ 18 O sw-ice , a proxy for salinity) and sea surface temperature (SST) records from IMAGES Core MD06-3047B recovered from the northern WTP to investigate the hydroclimate in the WTP during the last 700 kyr. Planktic foraminifera Globigerinoides ruber based Mg/Ca-SST and δ 18 O sw-ice records reveal distinct glacial-interglacial variations. The δ 18 O sw-ice record shows that our study site is characterized by fresher conditions and thus higher precipitation during glacials than during interglacials. Our results are in good agreement with numerical model simulations, which also suggest a glacial freshening in the northern WTP. A glacial freshening of the WTP is most likely related to a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC) and/or exposure of the Maritime Continent due to sea level changes which resulted in an enhanced Pacific Walker circulation and/or southward shift of the ITCZ. In addition, SST records from the WTP and the eastern tropical Pacific (ETP) suggest an enhanced tropical Pacific zonal SST gradient and thus La Niña-like conditions, which may have contributed to fresher conditions in the WTP during glacial periods. Besides, our rainfall record is in antiphase to the Chinese speleothem δ 18 O records, supporting the idea of an opposing trend in precipitation changes between the WTP and East Asia on orbital timescales. [ABSTRACT FROM AUTHOR]

Climate phenomena like the monsoon system, El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are interconnected via various feedback mechanisms and control the climate of the Indian Ocean and its surrounding continents on various timescales. The eastern tropical Indian Ocean is a key area for the interplay of these phenomena and for reconstructing their past changes and forcing mechanisms. Here we present records of upper ocean thermal gradient, thermocline temperatures (TT) and relative abundances of planktic foraminifera in core SO 189-39KL taken off western Sumatra (0°47.400′ S, 99°54.510′ E) for the last 8 ka that we use as proxies for changes in upper ocean structure. The records suggest a deeper thermocline between 8 ka and ca 3 ka compared to the late Holocene. We find a shoaling of the thermocline after 3 ka, most likely indicating an increased occurrence of upwelling during the late Holocene compared to the mid-Holocene which might represent changes in the IOD-like mean state of the Indian Ocean with a more negative IOD-like mean state during the mid-Holocene and a more positive IOD-like mean state during the past 3 ka. This interpretation is supported by a transient Holocene climate model simulation in which an IOD-like mode is identified that involves an insolation-forced long-term trend of increasing anomalous surface easterlies over the equatorial eastern Indian Ocean. [ABSTRACT FROM AUTHOR]