Plio-Pleistocene productivity reconstructions in the Indian Monsoon region

The Indian Summer Monsoon (ISM) brings seasonal winds and rains to the Indian subcontinent. The winds cause surface ocean mixing in the southern Bay of Bengal (BoB), bringing nutrients to the surface that fuel ocean productivity and export of carbon to the sea floor. To improve the understanding of the role of monsoon winds in low-latitude surface ocean productivity and carbon export, the Plio-Pleistocene interval is ideal because boundary conditions were evolving. We aim to reconstruct variability in surface ocean and export productivity across the late-Pliocene and early-Pleistocene in response to ISM wind-driven mixing in the southern BoB. Here we analyse sediments from IODP Site U1443 (Exp. 353) from ~1.9-2.8 million years ago. A new benthic oxygen isotope (δ18O) stratigraphy (at ~3-thousand-year resolution) and age model tied to the global benthic δ18O stack are presented. We utilise these sediments to obtain bulk sediment X-ray fluorescence (XRF) core scanning elemental data and coccolithophore assemblages to infer changes in summer monsoon runoff and surface ocean productivity influenced by monsoon wind strength. We use % Florisphaera profunda (coccolithophore assemblage) along with bulk sediment XRF Br as productivity indicators and a “terrigenous” bulk sediment XRF elemental composition stack as a wind strength and runoff indicator. We observe increased productivity during glacials (MIS 96, 98, 100), coinciding with increased terrigenous input. This observation is coherent with previous low-latitude productivity records from the equatorial Pacific. However, in contrast to equatorial Pacific productivity records, influenced by obliquity, our southern BoB records show robust surface productivity (% F. profunda) and summer monsoon runoff (terrigenous stack) peaks in both the obliquity and precession bands. We will discuss linkages between monsoon wind and runoff across the Plio-Pleistocene in context with other monsoon records

The Middle to Late Miocene “Carbonate Crash” in the Equatorial Indian Ocean

We integrate benthic foraminiferal stable isotopes, X‐ray fluorescence elemental ratios, and carbonate accumulation estimates in a continuous sedimentary archive recovered at International Ocean Discovery Program Site U1443 (Ninetyeast Ridge, Indian Ocean) to reconstruct changes in carbonate deposition and climate evolution over the interval 13.5 to 8.2 million years ago. Declining carbonate percentages together with a marked decrease in carbonate accumulation rates after ~13.2 Ma signal the onset of a prolonged episode of reduced carbonate deposition. This extended phase, which lasted until ~8.7 Ma, coincides with the middle to late Miocene carbonate crash, originally identified in the eastern equatorial Pacific Ocean and the Caribbean Sea. Interocean comparison reveals that intense carbonate impoverishment at Site U1443 (~11.5 to ~10 Ma) coincides with prolonged episodes of reduced carbonate deposition in all major tropical ocean basins. This implies that global changes in the intensity of chemical weathering and riverine input of calcium and carbonate ions into the ocean reservoir were instrumental in driving the carbonate crash. An increase in U1443 Log (Ba/Ti) together with a change in sediment color from red to green indicate a rise in organic export flux to the sea floor after ~11.2 Ma, which predates the global onset of the biogenic bloom. This early rise in export flux from biological production may have been linked to increased advection of nutrients and intensification of upper ocean mixing, associated with changes in the seasonality and intensity of the Indian Monsoon

Enhanced Late Miocene Chemical Weathering and Altered Precipitation Patterns in the Watersheds of the Bay of Bengal Recorded by Detrital Clay Radiogenic Isotopes

The late Miocene was a period of declining CO2 levels and extensive environmental changes, which likely had a large impact on monsoon strength as well as on the weathering and erosion intensity in the South Asian Monsoon domain. To improve our understanding of these feedback systems, detrital clays from the southern Bay of Bengal (International Ocean Discovery Program Site U1443) were analyzed for the radiogenic isotope compositions of Sr, Nd, and Pb to reconstruct changes in sediment provenance and weathering regime related to South Asian Monsoon rainfall from 9 to 5 Ma. The 100 kyr resolution late Miocene to earliest Pliocene record suggests overall low variability in the provenance of clays deposited on the Ninetyeast Ridge. However, at 7.3 Ma, Nd and Pb isotope compositions indicate a switch to an increased relative contribution from the Irrawaddy River (by ∼10%). This shift occurred during the global benthic δ13C decline, and we suggest that global cooling and increasing aridity resulted in an eastward shift of precipitation patterns leading to a more focused erosion of the Indo‐Burman Ranges. Sr isotope compositions were decoupled from Nd and Pb isotope signatures and became more radiogenic between 6 and 5 Ma. Grassland expansion generating thick, easily weatherable soils may have led to an environment supporting intense chemical weathering, which is likely responsible for the elevated detrital clay 87Sr/86Sr ratios during this time. This change in Sr isotope signatures may also have contributed to the late Miocene increase of the global seawater Sr isotope composition

Secular and orbital-scale variability of equatorial Indian Ocean summer monsoon winds during the late Miocene

In the modern northern Indian Ocean, biological productivity is intimately linked to near-surface oceanographic dynamics forced by the South Asian, or Indian, monsoon. In the late Pleistocene, this strong seasonal signal is transferred to the sedimentary record in the form of strong variance in the precession band (19–23 kyr), because precession dominates low-latitude insolation variations and drives seasonal contrast in oceanographic conditions. In addition, internal climate system feedbacks (e.g. ice-sheet albedo, carbon cycle, topography) play a key role in monsoon variability. Little is known about orbital-scale monsoon variability in the pre-Pleistocene, when atmospheric CO2 levels and global temperatures were higher. In addition, many questions remain open regarding the timing of the initiation and intensification of the South Asian monsoon during the Miocene, an interval of significant global climate change that culminated in bipolar glaciation. Here, we present new high-resolution (<1 kyr) records of export productivity and sediment accumulation from International Ocean Discovery Program Site U1443 in the southernmost part of the Bay of Bengal spanning the late Miocene (9 to 5 million years ago). Underpinned by a new orbitally tuned benthic isotope stratigraphy, we use X-ray fluorescence-derived biogenic barium variations to discern productivity trends and rhythms. Results show strong eccentricity-modulated precession-band productivity variations throughout the late Miocene, interpreted to reflect insolation forcing of summer monsoon wind strength in the equatorial Indian Ocean. On long timescales, our data support the interpretation that South Asian monsoon winds were already established by 9 Ma in the equatorial sector of the Indian Ocean, with no apparent intensification over the latest Miocene

The Middle to Late Miocene “Carbonate Crash” in the Equatorial Indian Ocean

金沢大学理工研究域地球社会基盤学系We integrate benthic foraminiferal stable isotopes, X-ray fluorescence elemental ratios, and carbonate accumulation estimates in a continuous sedimentary archive recovered at International Ocean Discovery Program Site U1443 (Ninetyeast Ridge, Indian Ocean) to reconstruct changes in carbonate deposition and climate evolution over the interval 13.5 to 8.2 million years ago. Declining carbonate percentages together with a marked decrease in carbonate accumulation rates after ~13.2 Ma signal the onset of a prolonged episode of reduced carbonate deposition. This extended phase, which lasted until ~8.7 Ma, coincides with the middle to late Miocene carbonate crash, originally identified in the eastern equatorial Pacific Ocean and the Caribbean Sea. Interocean comparison reveals that intense carbonate impoverishment at Site U1443 (~11.5 to ~10 Ma) coincides with prolonged episodes of reduced carbonate deposition in all major tropical ocean basins. This implies that global changes in the intensity of chemical weathering and riverine input of calcium and carbonate ions into the ocean reservoir were instrumental in driving the carbonate crash. An increase in U1443 Log (Ba/Ti) together with a change in sediment color from red to green indicate a rise in organic export flux to the sea floor after ~11.2 Ma, which predates the global onset of the biogenic bloom. This early rise in export flux from biological production may have been linked to increased advection of nutrients and intensification of upper ocean mixing, associated with changes in the seasonality and intensity of the Indian Monsoon. ©2019. American Geophysical Union. All Rights Reserved.Embargo Period 6 month

Reconstructing Indian Monsoon Driven Productivity and Stratification Changes Across the Plio-Pleistocene

The Indian Summer Monsoon (ISM) brings seasonal winds and rains to the Indian subcontinent. The winds cause surface ocean mixing in the southern Bay of Bengal (BoB), bringing nutrients to the surface that fuel ocean productivity and export of carbon to the sea floor. To improve the understanding of the role of monsoon winds in low-latitude surface ocean productivity and carbon export, the Plio-Pleistocene interval is ideal because boundary conditions were evolving. This thesis uses a multiproxy approach to reconstruct variability in surface ocean and export productivity using coccolithophore species assemblage and bulk sediment composition, upper ocean stratification changes using planktic foraminifera geochemical proxies, and nutrient conditions using coccolithophore species assemblage across the late-Pliocene and early-Pleistocene in response to variability in ISM wind-driven mixing in the southern BoB. We present a new orbitally tuned stable oxygen isotope (δ18O) benthic foraminifera age-model and new orbital-resolution records of southern Bay of Bengal productivity (percentage F. profunda), terrigenous input (bulk sediment chemical composition using X-Ray Fluorescence core scan data), upper ocean Temperature (sea surface temperature, SST, using surface dwelling planktic species Globigerinoides ruber sensu stricto and subsurface temperature using planktic species Neogloboquadrina dutertrei) and Stratification (from coupled planktic foraminifera trace element and stable isotopes composition), and coccolithophore assemblage and morphological changes. We find that productivity records are dominated by obliquity frequencies, unlike during the Pleistocene and the late Miocene at the same site when it has been shown to be precession dominated. We suggest that a combination of insolation and longitudinal SST gradients, causing increased monsoon winds, and therefore wind driven mixing, bring nutrients to the surface. This study explains that the presence of obliquity signals in the productivity record at this low-latitude monsoon wind influenced site, which could be expected to be precession dominated, could be due to the supply of nutrients from the high latitudes to the low-latitudes during glacials. We find that SST and stratification do not show obliquity cycles and are instead driven by precession, supporting the suggestion of some non-monsoon (high latitude) forcing of productivity to give obliquity signals. We suggest that changes in coccolithophore morphology are caused by a combination of decreased nutrient availability and increased seasonality, caused by a strengthening winter monsoon after the intensification of northern hemisphere glaciation

Drivers of late Miocene tropical sea surface cooling: A new perspective from the equatorial Indian Ocean

International audienceDuring the late Miocene, global cooling occurred alongside the establishment of near-modern terrestrial and marine ecosystems. Significant (3 to 5 °C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at mid to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital-resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9 to 5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. Our SST record reveals a 3.2 °C decrease from 7.4 to 5.8 Ma, significantly increasing previous estimates of late Miocene tropical cooling. Analysis of orbital-scale variability shows that before the onset of cooling, SST variations were dominated by precession band (19-23 kyr) variability, whereas tropical temperature became highly sensitive to obliquity (41 kyr) after 7.5 Ma, suggesting an increase in high latitude forcing. We compare a revised global SST database with new paleoclimate model simulations and show that a pCO2 decrease from 560 ppm to 300 ppm, in the range suggested by pCO2 proxy records, could explain most of the late Miocene sea surface cooling observed at Site U1443. Using our new Site U1443 record as representative of tropical SST evolution, estimated meridional SST gradients suggest a much more modest increase over the late Miocene than previously suggested, in agreement with modelled meridional SST gradients

Climatic simulation from the equatorial Indian Ocean

During the late Miocene, global cooling occurred alongside the establishment of near-modern terrestrial and marine ecosystems. Significant (3 to 5 °C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at mid to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital-resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9 to 5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. To test if an atmospheric pCO2 decrease may have driven this cooling, we also present new paleoclimate model simulations under three atmospheric pCO2 scenarios (300 ppm, 420 ppm and 560 ppm; in the range suggested by existing pCO2 proxy records). The climatic simulation presents the effect of late Miocene pCO2 on Sea surfaces Temperatures. This dataset contains sea surface temperature outputs from modeling experiments with variable CO2 levels and a late Miocene paleogeography (Sarr et al., in review). The simulations have been run using the IPSL-CM5A2 General Circulation Model (Sepulchre et al. 2020 - GMD). It includes 3 simulations at 300, 420 and 560 ppm respectively. Data are monthly averages over the last 100 years of the simulations. Files have curvilinear coordinates (nav_lon, nav_lat)

Mg/Ca ratios measured on planktic foraminifera species and reconstructed Sea Surface Temperature (SST) from IODP Site 353-U1443

During the late Miocene, global cooling occurred alongside the establishment of near-modern terrestrial and marine ecosystems. Significant (3 to 5 °C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at mid to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital-resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9 to 5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. To test if an atmospheric pCO2 decrease may have driven this cooling, we also present new paleoclimate model simulations under three atmospheric pCO2 scenarios (300 ppm, 420 ppm and 560 ppm; in the range suggested by existing pCO2 proxy records). The data contains Mg/Ca ratios (mmol/mol) measured in tests of a planktic foraminifera species living in the mixed layer (Trilobatus trilobus), together with reconstructed Sea Surface Temperatures. Samples are from the revised shipboard splice from Site U1443 (equatorial Indian Ocean) retrieved during International Ocean Discovery Program Expedition 353. Data span the late Miocene-earliest Pliocene (9 to 5 Ma) with a mean resolution of 5.5 kyr and allow reconstruction of SSTs on secular to orbital timescales. Sea Surface Temperatures are reconstructed with the Dekens et al., (2002) T. sacculifer calibration equation from Pacific Ocean, including a basin-specific dissolution correction. Correction for variation of Mg/Ca ratio of seawater are also calculated following the scenario from Higgins & Schrag (2015) and the approach from Tierney et al. (2019). ±1σ and ±2σ uncertainties linked to analytical, calibration, and age model errors are estimated via a bootstrap Monte Carlo procedure using the Paleo-Seawater Uncertainty Solver (PSU solver, Thirumalai et al., 2016) in Matlab