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Climate and topography change the characteristics of the Asian monsoon over millions of years. These changes affect the region’s climate and topography, and the cycle continues.

International audience

International audience; During the late Miocene (~11-5 Ma), atmospheric pCO2may have been 1 to 3 times pre-industrial values1,2, and mean global temperatures were likely 3−4°C higher than today with reduced latitudinal gradients3. Major ecosystem changes that took place during the late Miocene (e.g., ref 4 and refs therein) are consistent with falling global temperatures and decreasing pCO2, but proxy evidence for these physical climate changes remains sparse because few continuous, orbital-resolution palaeoclimate records cover this interval. Recently, geographically widespread and major (~5-10°C) late Miocene sea surface cooling was revealed for the mid-and high-latitudes based on the Uk'37 sea surface temperature (SST) proxy3. However, existing tropical SST records suggest only minor cooling in this interval (1-2°C). Here we present new multi-proxy geochemical records of surface and deep ocean variability from IODP Site U1443, drilled in the equatorial Indian Ocean (5°N, 90°E) during IODP Expedition 353, which spanthe interval 9 to 5 Ma. Surface ocean records from Site U1443 have the potential to resolve SST change at a “warm pool” tropical site, whilst also providing insight into the long-term evolution in the Indian monsoon subsystem. SST estimates derived from Mg/Ca ratios in the surface-dwelling planktic foraminifer Trilobatus trilobusshow ~4°C of cooling (3.8 -2.5 mmol/mol Mg/Ca) between 8 and 6 Ma. Following a SST minimum at 5.8-6 Ma, reconstructed temperatures increase from 5.8 to 5 Ma, consistent with published extratropical SST records. SST estimates combined with paired foraminiferal δ18O measurements reveal a ~1.5‰ decrease in local seawater δ18O between 8 and 6 Ma, and a subsequent increase between 6 and 5 Ma. Consistent with records from other sites, Site U1443 benthic δ18O data show no long-term increase concurrent with the SST decrease. We compare our results with late Miocene simulations using the IPSL-CM5 earth system model, exploring both pCO2and paleogeographic effects on regional paleoceanography.

We reconstructed the variability of the Earth's strongest hydrological system, the Indian monsoon, over the interval 6.24 to 4.91 Ma at International Ocean Discovery Program (IODP) Expedition 353 Site U1448 in the Andaman Sea. We integrated high-resolution benthic and planktic foraminiferal carbon and oxygen isotopes with Mg/Ca measurements of the mixed layer foraminifer Trilobatus sacculifer to reconstruct the isotopic composition of seawater (δ18Osw) and the gradient between planktic and benthic foraminiferal δ13C. A prominent increase in mixed layer temperatures of ~4°C occurred between 5.55 and 5.28 Ma, accompanied by a change from precession- to obliquity-driven variability in planktic δ18O and δ18Osw. We suggest that an intensified cross-equatorial transport of heat and moisture, paced by obliquity, led to increased summer monsoon precipitation during warm stages after 5.55 Ma. Transient cold stages were characterized by reduced mixed layer temperatures and summer monsoon failure, thus resembling late Pleistocene stadials. In contrast, an overall cooler background climate state with a strengthened biological pump prevailed prior to 5.55 Ma. These findings highlight the importance of internal feedback processes for the long-term evolution of the Indian monsoon.

The Andaman Sea is ideally situated to assess the sensitivity of the Indian Monsoon to insolation forcing and to changes in climate boundary conditions such as global ice volume and greenhouse gas concentrations on a warmer-than-present Earth. Sediment archives from this region record the monsoonal discharge of water and sediment from mountainous areas at the southern slope of the Eastern Himalayas, the western slopes of the Arakan Mountains and Indo-Burman Ranges and from the catchment of the Irrawaddy, Sittang and Salween Rivers. New sediment cores recovered during International Ocean Discovery Program (IODP) Expedition 353 (iMonsoon) provide the first complete millennial-scale resolution record of the Indian Monsoon over the late Neogene, when major changes in the Indian and East-Asian monsoonal subsystems occurred. Initial results from sediment natural gamma ray core-logging and foraminiferal stable isotope analyses indicate that a long-term increase in physical weathering and erosion coincided with a prolonged late Miocene cooling trend between ~7 and 5.5 Ma. Monsoonal erosion remained intense during the subsequent warming episode between 5.5 and 5.3 Ma, probably associated with a northward shift of the monsoonal rain belt. Ocean mixed layer oxygen isotope records indicate freshening of the Andaman Sea during late Miocene warm isotope stages as during Pleistocene interglacials. Knowledge of this past evolution of freshwater and sediment discharge from one of the core areas of Indian Monsoon precipitation is essential to better predict trends and consequences of monsoonal spatial variability with future climate change.

The distribution of benthic foraminifera of the Uvigerinidae family, which are abundant in middle to upper Pleistocene samples from the Yamoto Basin in the Japan Sea, have been analyzed quantitatively and used for paleoceanographic interpretations. Twenty-three Uvigerinidae species are reported from IODP Site U1426, 6 of which are present in at least three or more samples with 10% or more abundance and considered as dominant species. The abundance and ecological preferences of dominant Uvigerinidae species, comprised of Uvigerina mediterranea, Uvigerina yabei, Uvigerina sp.1, Uvigerina peregrina suggest dysoxic conditions, with high influx of organic matter between 775 and 475 ka in response to the enhanced flow of the Tsushima Warm Current. The end of MIS 13 marked the onset of more extreme glacial interglacial climate cycles in the Japan Sea. Glacial interglacial alternations of Uvigerina peregrina and Trifarina angulosa from 475 ka to the Holocene indicate major fluctuations in circulation and productivity within this semi-enclosed basin. Both U. peregrina and T. angulosa respond to 100 kyr variability, reflecting the paleoclimatic shift and global ice volume changes associated with the Middle Pleistocene Transition.

The Indonesian Throughflow (ITF), the sole conduit between the Pacific and Indian Oceans, regulates the heat and fresh water budgets between these oceans and plays a key role as the ‘warm water pathway’ in the global thermohaline circulation. In this study, we reconstructed a 25-kyr history of the ITF thermocline, based on well-dated, high-resolution (~200 yrs) records of Pulleniatina obliquiloculata δ18O and Mg/Ca from Core SO217-18515 (3.63°S, 119.54°E; water depth: 688 m), retrieved from the southern Makassar Strait. We integrated these new records with data from offshore Luzon (MD98-2188; 14.82°N, 123.49°E; water depth: 730 m), from the center of the Western Pacific Warm Pool (WPWP) (3cBX; 8.02°N, 139.6°E; water depth: 2829 m) and from the Timor Sea (MD01-2378; 13.08°S, 121.79°E; water depth: 1783 m). Both sea surface and thermocline temperatures decreased and their difference (ΔT) increased in the southern Makassar Strait and Timor Sea between 11 and 2.5 ka, corresponding to enhanced El Nino activity and dominance of East Asian winter monsoon over summer monsoon. In contrast, sea surface and thermocline temperatures increased and ΔT decreased during the last ~2.5 kyrs concomitantly with declining El Nino activity and relative weakening of the East Asian winter monsoon. Our results additionally show that ice volume corrected seawater δ18O of both sea surface and thermocline in Cores SO217-18515 and MD01-2378 co-vary with Borneo stalagmite δ18O, a proxy of regional precipitation in the WPWP, thus indicating vertical mixing of upper ocean waters along the ITF pathway. We propose that changes in the state of the El Nino-Southern Oscillation and in East Asian monsoon related to regional precipitation exerted a major influence on the ITF thermocline profile during the Holocene and largely controlled changes in ITF thermocline temperature throughout the last 25 kyrs.

Integrated Ocean Drilling Program Expedition 346 “Asian Monsoon” obtained sediment successions at seven sites in the Japan Sea (Sites U1422–U1427 and U1430) and at two closely located sites in the northern East China Sea (Sites U1428 and U1429). The Quaternary sediments of the Japan Sea are characterized by centimeter- to decimeter-scale dark–light alternations at all sites deeper than 500 m water depth. The sedimentary records from these sites allow an investigation of the regional environmental response to global climate change, including changes in the Asian Monsoon and eustatic sea level. However, the discontinuous occurrence of calcareous microfossils in the deep-sea sediments and their distinct isotope signature that deviates from standard marine δ18O records do not permit the development of a detailed stable isotope stratigraphy for Japan Sea sediments. Here, we present the tephrostratigraphy for the two southernmost sites drilled in the Japan Sea (Sites U1426 and U1427) and for one site drilled in the East China Sea (Site U1429) along with the benthic δ18O isotope stratigraphy for the shallower Site U1427 and the East China Sea Site U1429. Eighteen tephra layers can be correlated between sites using the major-element composition and morphology of volcanic glass shards, and the compositions of grains and heavy minerals. Tephra correlations show that negative δ18O peaks in the Japan Sea correspond to positive glacial maxima peaks in the East China Sea. Using this integrated stratigraphic approach, we establish an orbital-scale age model at Site U1427 for the past 1.1 Myr. The correlation of tephra layers between the shallower Site U1427 (330 m below sea level: mbsl) and the deeper Site U1426 (903 mbsl) in the southern Japan Sea provides the opportunity for further age constraints. Our results show that alternations in sediment color at Sites U1426 and U1427 can be correlated for the past 1.1 Myr with minor exceptions. Thus, the stable isotope stratigraphy established at the shallower Site U1427 can be correlated to Site U1426, and in turn to all sites drilled during Expedition 346, based on correlations of dark–light layering.

We reconstructed the orbital-scale variability of deep-water mass properties in the equatorial Indian Ocean during the late early to late Miocene (17.9 to 8.2 Ma) using benthic foraminiferal oxygen (δ18O) and carbon (δ13C) isotope records, in combination with X-ray fluorescence scanner elemental records at International Ocean Discovery Program (IODP) Site U1443. These records suggest that hyperthermal-like features (negative excursions in δ18O and δ13C coupled to suboxic and acidic conditions in the deep ocean) developed at eccentricity maxima during the Miocene Climatic Optimum (MCO; 16.9 to 14.7 Ma) on an almost ice-free Earth. Elevated Mn and U concentrations indicate that bottom and pore waters in the Indian Ocean became progressively suboxic during the later phase of the MCO. A fundamental change in deep-water circulation occurred after the expansion of the East Antarctic Ice Sheet during the Middle Miocene Climatic Transition (MMCT; 14.7 to ~13.8 Ma). Stepwise increases in δ18O at ~13.8 and ~13.1 Ma concurred with dampening of deep-water δ18O and δ13C variability, as Antarctic ice cover expanded and became more stable. A marked improvement in deep-water oxygenation after ~13.5 Ma coincided with the end of the last δ13C maximum of the Monterey Excursion. Carbonate dissolution in the equatorial Indian Ocean intensified and prolonged episodes of reduced carbonate deposition during the Carbonate Crash interval (~12.6 to 8.6 Ma) were not restricted to eccentricity maxima as during the MCO. Rising Mn and U concentrations after ~9 Ma indicate decreased oxygenation of bottom and pore waters, associated with the onset of the Biogenic Bloom in the Indian Ocean.

We present a high-resolution (~20 to 100 years temporal resolution) reconstruction of hydrological changes in the Makassar Strait over the last 14 kyr from Core SO217-18517 retrieved off the Mahakam Delta (1°32.198′S, 117°33.756′E; 698 m water depth) during the SO217 Makassar-Java Cruise. Sea surface temperatures, based on Mg/Ca of Globigerinoides ruber and alkenone UK′37, and seawater δ18O reconstructions, based on G. ruber δ18O and Mg/Ca, in combination with sortable silt grain size measurements and X-ray fluorescence (XRF) core scanner derived elemental data provide evidence for increased precipitation during the Bolling-Allerod (BA) and early Holocene and for warmer and more saline surface waters and a decrease in the intensity of the Indonesian Throughflow (ITF) during the Younger Dryas (YD). XRF derived Log (Zr/Rb) records, sortable silt data and increased sedimentation rates indicate decreased winnowing, interpreted as a slowdown of the ITF thermocline flow during the YD. We attribute this decline in ITF intensity to slowdown of the Atlantic meridional overturning circulation during the YD. We suggest that changes in Makassar Strait surface hydrology during this interval of Northern Hemisphere cooling and Southern Hemisphere warming were related to a southward displacement of the Intertropical Convergence Zone.

We investigated the onset and development of Cretaceous Oceanic Anoxic Event 2 (OAE2) in a newly drilled core (SN degrees 4) from the Tarfaya Basin (southern Morocco), where this interval is unusually expanded. High-resolution (centimeter-scale equivalent to centennial) analysis of bulk organic and carbonate stable isotopes and of carbonate and organic carbon content in combination with XRF scanner derived elemental distribution reveal that the ocean-climate system behaved in a highly dynamic manner prior to and during the onset of OAE2. Correlation with the latest orbital solution indicates that the main carbon isotope shift occurred during an extended minimum in orbital eccentricity (similar to 400 kyr cycle). Shorter-term fluctuations in carbonate and organic carbon accumulation and in sea level related terrigenous discharge were predominantly driven by variations in orbital obliquity. Negative excursions in organic and carbonate delta C-13 preceded the global positive delta C-13 shift marking the onset of OAE2, suggesting injection of isotopically depleted carbon into the atmosphere. The main delta C-13 increase during the early phase of OAE2 in the late Cenomanian was punctuated by a transient plateau. Maximum organic carbon accumulation occurred during the later part of the main delta C-13 increase and was associated with climate cooling events, expressed as three consecutive maxima in bulk carbonate delta O-18. The extinctions of the thermocline dwelling keeled planktonic foraminifers Rotalipora greenhornensis and Rotalipora cushmani occurred during the first and last of these cooling events and were likely associated with obliquity paced, ocean-wide expansions, and intensifications of the oxygen minimum zone, affecting their habitat space on a global scale.

We investigate changes in benthic foraminiferal assemblages and estimate bottom water temperatures (BWT), based on Mg/Ca ratios of Oridorsalis umbonatus , over three selected intervals of major climate change during the late early to middle Miocene at Integrated Ocean Drilling Program (IODP) Sites U1337 (4463 m water depth) and U1338 (4200 m water depth), eastern equatorial Pacific Ocean. The targeted intervals are: (1) the onset of the Miocene Climatic Optimum (MCO; at ~ 16.9 Ma) and associated carbon cycle perturbation (Site U1337); (2) the episode of peak warmth within the MCO centered at 15.6 Ma (Site U1338); (3) the major cooling step at ~ 13.8 Ma related to east Antarctic ice-sheet expansion (Site U1338). Assemblages from these three intervals are mainly composed of cosmopolitan, long ranging (e.g., Paleogene to recent), lower bathyal to abyssal species, including calcareous elongated forms that became extinct during the middle Pleistocene. The onset of the MCO and interval of peak warmth had little impact on benthic foraminiferal accumulation rates (BFAR), abundances and assemblage composition. A transient BWT warming of 2.6 °C at the onset of the interval of peak warmth concurred with a benthic stable oxygen isotope (δ 18 O) decrease of 1‰, suggesting that the δ 18 O decrease was largely controlled by BWT changes. A substantial increase in BFAR and in the abundances of Epistominella exigua and epifaunal taxa after ~ 13.83 Ma coincides with increased opal accumulation rates, reflecting enhanced and seasonally pulsed organic carbon (C org ) flux to the seafloor during the global cooling step. During this interval, BWT decrease by 1.7 °C concurrent with a 1.0‰ increase in the stable oxygen isotope composition of seawater (δ 18 O sw ). Comparable trends at Ocean Drilling Program (ODP) Site 1146 in the South China Sea, with a 2.2 °C cooling and 0.9‰ increase in δ 18 O sw at ~ 13.8 Ma, further support that the benthic δ 18 O increase during the major cooling step was largely controlled by ice volume changes.

The stable isotope composition of planktonic foraminifera, Globigerinoides ruber (G. ruber), is a commonly used paleoenvironmental proxy. Globigerinoides ruber has two primary morphotypes, sensu stricto and sensu lato, which are differentiated by taxonomic criteria and, in some instances, stable isotopic values. Previous studies have reached differing conclusions surrounding the potential bias caused by the use of non-selective mixtures of morphotypes in paleoenvironmental reconstructions involving stable isotopes. Here, G. ruber s.s. and G. ruber s.l. from five time intervals within the last 85 ka, were analyzed from IODP Site U1429 sediments in the East China Sea. The oxygen isotopic compositions of these two morphotypes show statistically significant differences within all five intervals, with smaller s.s. values relative to s.l. values. The largest differences (0.77‰ to 0.82‰) are observed during interglacials (Holocene and Marine Isotopic Stage 5.1) with smaller differences during Marine Isotopic Stage 3 (0.45‰ to 0.56‰) and the Last Glacial Maximum (0.21‰). The isotopic offset between s.s. and s.l. is consistent with a deeper (colder) calcification depth of the s.l. morphotypes in the East China Sea. The decreased offset between the two morphotypes with increased glacial conditions may reflect decreased water column stratification during glacial intervals relative to interglacial intervals. According to these results, using a non-selective mixture of G. ruber morphotypes in paleoenvironmental reconstructions is inadvisable in this region.

Precipitation is the most important variable of Indonesian climate, yet there are substantial uncertainties about past and future hydroclimate dynamics over the region. This study explores vegetation and rainfall and associated changes in atmospheric circulation during the past 26,000 years in Wallacea, a biogeographical area in central Indonesia, wedged between the Sunda and Sahul shelves and known for its exceptionally high rainforest biodiversity. We use terrestrial plant biomarkers from sediment cores retrieved from Mandar Bay, off west Sulawesi, to reconstruct changes in Wallacean vegetation and climate since the Last Glacial Maximum (LGM). Enriched leaf wax carbon isotope (δ13Cwax) values recorded in Mandar Bay during the LGM, together with other regional vegetation records, document grassland expansion, implying a regionally dry, and possibly more seasonal, glacial climate. Depleted leaf wax deuterium isotope (δDwax) values in Mandar Bay during the LGM, and low reconstructed precipitation isotope compositions from nearby sites, reveal an intensified Austral-Asian summer monsoon circulation and a southward shift of the mean position of the Intertropical Convergence Zone, likely due to strong southern hemisphere summer insolation and the presence of large northern hemisphere ice sheets. Mandar Bay δ13Cwax was anti-correlated with δDwax during the LGM and the last deglaciation, but was positively correlated during most of the Holocene, indicating time-varying controls on the isotopic composition of rainfall in this region. The inundation event of the Sunda Shelf and in particular the opening of the Java Sea and Karimata Strait between 9.4 and 11.1 thousand years ago might have provided new moisture sources for regional convection and/or influenced moisture source trajectories, providing the trigger for shifts in atmospheric circulation and the controls on precipitation isotope compositions from the LGM to the Holocene.