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Paleoceanographic studies often rely on abundance changes in microfossil species, with little consideration for characteristics such as organism size, which may also be related to environmental changes. Using a tropical Indian Ocean (TIO) core‐top data set, we test the Optimum size‐hypothesis (OSH), investigating whether relative abundance or environmental variables are better descriptors of planktonic foraminifera species' optimum conditions. We also investigate the environmental drivers of whole‐assemblage planktonic foraminiferal test size variation in the TIO. We use an automated imaging and sorting system (MiSo) to identify planktonic foraminiferal species, analyze their morphology, and quantify fragmentation rate using machine learning techniques. Machine model accuracy is confirmed by comparison with human classifiers (97% accuracy). Data for 33 environmental parameters were extracted from modern databases and, through exploratory factor analysis and regression models, we explore relationships between planktonic foraminiferal size and oceanographic parameters in the TIO. Results show that the size frequency distribution of most planktonic foraminifera species is unimodal, with some larger species showing multimodal distributions. Assemblage size95/5 (95th percentile size) increases with increasing species diversity, and this is attributed to vertical niche separation induced by thermal stratification. Our test for the OSH reveals that relative abundance is not a good predictor of species' optima and within‐species size95/5 response to environmental parameters is species‐specific, with parameters related to carbonate ion concentration, temperature, and salinity being primary drivers. At the species and assemblage levels, our analyses indicate that carbonate ion concentration and temperature play important roles in determining size trends in TIO planktonic foraminifera. Plain Language Summary: In core‐top samples from the tropical Indian Ocean (TIO), we investigate the optimum size‐hypothesis, testing whether species' relative abundance or environmental parameter(s) are better descriptors of planktonic foraminifera species' optimum conditions. Further, we investigate the main environmental drivers of size variations in planktonic foraminifera at the assemblage‐level, given that temperature has been reported to primarily drive assemblage size trends. We use a state‐of‐the‐art machine (MiSo) to automatically identify planktonic foraminiferal species, analyze their size, and quantify fragmentation using machine learning techniques. When compared to identification carried out by human experts across 21 species, the machine classified the species accurately 97% of the time. The MiSo‐generated size data was similar to that by other researchers. The frequency distributions of the species' size spectra show that most species have distributions that form bell‐shaped curves. As species diversity increased, so did the assemblage size (95th percentile size); we attribute this observation to the effect of temperature‐dependent niche separation. We find that, in the TIO, environmental parameters are better descriptors of optimum conditions in planktonic foraminifera than relative abundance. Our results also reveal that size variation at the species and assemblage levels is mostly driven by ambient carbonate chemistry and temperature. Key Points: Optimum size‐hypothesis holds true in planktonic foraminifera if one considers the main parameters driving each species' size distributionSize variations in planktonic foraminifera are linked to species' niches and diversity does not increase with productivityWithin‐species size is driven by CO32− concentration, temperature, and salinity; assemblage size by CO32− concentration and temperature [ABSTRACT FROM AUTHOR]

During the late Miocene, global cooling occurred alongside the establishment of near‐modern terrestrial and marine ecosystems. Significant (3°C–5°C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at midlatitudes 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 (9–5 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 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 modeled meridional SST gradients. Plain Language Summary: The late Miocene is an interesting time period for paleoclimatologists because the Earth underwent important climatic and ecological changes that led to the establishment of our modern climate. An important cooling of global surface oceans was recorded by tracers used to reconstruct past temperature; however, the amplitude of this cooling in the tropics and the role of atmospheric carbon dioxide (CO2) in driving it are unclear. We present a new reconstruction of sea surface temperatures from the eastern equatorial Indian Ocean based on the temperature‐dependent ratio of magnesium to calcium measured in fossil shells of zooplankton (Foraminifera). Our results reveal a cooling (3.2°C) higher than previous estimates of tropical ocean cooling (1°C–2.5°C). To understand the role of atmospheric CO2 in driving this cooling, we simulated Miocene climate using a complex model and find that an atmospheric CO2 decrease from 560 to 300 ppm could explain most of the reconstructed surface ocean cooling. We also find that ocean surface temperature gradients between the tropics (using our new reconstruction) and the northern high latitudes shows a more modest increase over the late Miocene than suggested by previous studies, in agreement with new and existing climate model results. Key Points: New late Miocene orbital‐resolution Mg/Ca sea surface temperature record from the eastern equatorial Indian Ocean spanning 9–5 MaTropical cooling is >3°C; new model simulations suggest that a pCO2 decrease from 560 to 300 ppm could account for most of this coolingIncrease in meridional sea surface temperature gradients over the late Miocene more modest than previously suggested [ABSTRACT FROM AUTHOR]

New measurements of stable isotope vital effects in fossil coccoliths show a step increase in reliance of coccolithophore photosynthesis on active transport of dissolved bicarbonate in the late Miocene epoch, suggesting both a low threshold for adaptation of coccolithophores to carbon dioxide and a decrease in global carbon dioxide levels at that time. Phytoplankton cell size tracks atmospheric CO.sub.2 Coccolithophores, widely distributed in the marine plankton, are unique among algae in that they use carbon for both calcification and photosynthesis. In this study Clara Bolton and Heather Stoll use a model of cellular carbon fluxes to show that when carbon dioxide concentrations are low, these organisms will allocate carbon preferentially to photosynthesis rather than to calcification, particularly in larger cells. This is reflected in a difference between the isotopic signature of small and large coccoliths that diminishes at high levels of carbon dioxide. This pattern can be seen in the fossil record; the authors identify an isotopic divergence between small and large coccoliths at around 6 million years ago, and interpret this as a threshold response of the cells' carbon acquisition to a global decrease in carbon dioxide concentrations at the time. Coccolithophores are marine algae that use carbon for calcification and photosynthesis. The long-term adaptation of these and other marine algae to decreasing carbon dioxide levels during the Cenozoic era.sup.1 has resulted in modern algae capable of actively enhancing carbon dioxide at the site of photosynthesis. This enhancement occurs through the transport of dissolved bicarbonate (HCO.sub.3.sup.-) and with the help of enzymes whose expression can be modulated by variable aqueous carbon dioxide concentration, [CO.sub.2], in laboratory cultures.sup.2,3. Coccolithophores preserve the geological history of this adaptation because the stable carbon and oxygen isotopic compositions of their calcite plates (coccoliths), which are preserved in the fossil record, are sensitive to active carbon uptake and transport by the cell. Here we use a model of cellular carbon fluxes and show that at low [CO.sub.2] the increased demand for HCO.sub.3.sup.- at the site of photosynthesis results in a diminished allocation of HCO.sub.3.sup.- to calcification, which is most pronounced in larger cells. This results in a large divergence between the carbon isotopic compositions of small versus large coccoliths only at low [CO.sub.2]. Our evaluation of the oxygen and carbon isotope record of size-separated fossil coccoliths reveals that this isotopic divergence first arose during the late Miocene to the earliest Pliocene epoch (about 7-5 million years ago). We interpret this to be a threshold response of the cells' carbon acquisition strategies to decreasing [CO.sub.2]. The documented coccolithophore response is synchronous with a global shift in terrestrial vegetation distribution between 8 and 5 Myr ago, which has been interpreted by some studies as a floral response to decreasing partial pressures of carbon dioxide ([Formula omitted]) in the atmosphere.sup.4,5,6. We infer a global decrease in carbon dioxide levels for this time interval that has not yet been identified in the sparse [Formula omitted] proxy record.sup.7 but is synchronous with global cooling and progressive glaciations.sup.8,9., Author(s): Clara T. Bolton [sup.1] , Heather M. Stoll [sup.1] Author Affiliations: (1) Geology Department, University of Oviedo, Jesus Arias de Velasco S/N, 33005, Oviedo, Asturias, Spain, Main Coccolithophores are [...]

'Hyperthermals' are intervals of rapid, pronounced global warming known from six episodes within the Palaeocene and Eocene epochs (~65-34 million years (Myr) ago) (1-13). The most extreme hyperthermal was the [...]

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 CO 2 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. [ABSTRACT FROM AUTHOR]

Carbon isotope records from alkenone biomarkers (εp37:2) produced by haptophyte algae are frequently used for atmospheric CO2 paleobarometry, but this method has yielded inconsistent results during periods where CO2 variations are known independently. Recent syntheses of algal cultures have quantitatively demonstrated that εp37:2 indeed records CO2 information: εp37:2 increases as aqueous CO2 concentrations increase relative to carbon demand. However, interpretations of εp37:2 are complicated by irradiance, where higher irradiance yields higher εp37:2. Here we examine the roles of physiology and environment in setting εp37:2 in the ocean. We compile water‐column and sediment core‐top εp37:2 data and add new core‐top measurements, including estimates of cell sizes and growth rates of the alkenone‐producing population. In support of culture studies, we find irradiance to be a key control on εp37:2 in the modern ocean. We test a culture‐derived model of εp37:2 and find that the quantitative relationships calibrated in culture experiments can be used to predict εp37:2 in sediment samples. In water‐column samples, the model substantially overestimates εp37:2, largely resulting from higher irradiance at the depth of sample collection than the integrated light conditions under which the sampled biomass was produced and vertically mixed to the collection depth. We argue that the theory underpinning the conventional diffusive alkenone carbon isotope fractionation model, including the "b" parameter, is not supported by field data and should not be used to reconstruct past CO2 changes. Future estimates of CO2 from εp37:2 should use empirical or mechanistic models to quantitatively account for irradiance and cell size variations. Key Points: Irradiance is a primary control on alkenone carbon isotope fractionation in the natural environmentCulture‐calibrated model can predict alkenone carbon isotope fractionation in core‐top sediment samplesDiffusive model "b" value is not correlated with growth rate: diffusive model should not be used for paleobarometry [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 as 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 variability of the monsoon 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 Bay of Bengal spanning the late Miocene and earliest Pliocene (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. Our data 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, with no apparent intensification over the late Miocene. [ABSTRACT FROM AUTHOR]

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. Key Points: Extended episode of low carbonate accumulation (carbonate crash) between ~13.2 and ~8.7 Ma in the equatorial Indian OceanMost intense phase of carbonate crash in Indian Ocean coincides with reduced carbonate deposition in Atlantic and Pacific OceansEarly onset of biogenic bloom at ~11.2 Ma in Indian Ocean related to intensification of Indian Monsoon [ABSTRACT FROM AUTHOR]

The North Atlantic Current (NAC) transports warm salty water to high northern latitudes, with important repercussions for ocean circulation and global climate. A southward displacement of the NAC and Subarctic Front, which separate subpolar and subtropical water masses, is widely suggested for the Last Glacial Maximum (LGM) and may have acted as a positive feedback in glacial expansion at this time. However, the role of the NAC during the intensification of Northern Hemisphere glaciation (iNHG) at ~3.5 to 2.5 Ma is less clear. Here we present new records from Integrated Ocean Drilling Program Site U1313 (41°N) spanning ~2.8–2.4 Ma to trace the influence of Subarctic Front waters above this mid‐latitude site. We reconstruct surface and permanent pycnocline temperatures and seawater δ18O using paired Mg/Ca‐δ18O measurements on the planktic foraminifers Globigerinoides ruber and Globorotalia crassaformis and determine abundances of the subpolar foraminifer Neogloboquadrina atlantica. We find that the first significant glacial incursions of Subarctic Front surface waters above Site U1313 did not occur until ~2.6 Ma. At no time during our study interval was (sub)surface reorganization in the midlatitude North Atlantic analogous to the LGM. Our findings suggest that LGM‐like processes sensu stricto cannot be invoked to explain interglacial‐glacial cycle amplification during iNHG. They also imply that increased glacial productivity at Site U1313 during iNHG was not only driven by southward deflections of the Subarctic Front. We suggest that nutrient injection from cold‐core eddies and enhanced glacial dust delivery may have played additional roles in increasing export productivity in the midlatitude North Atlantic from 2.7 Ma. Key Points: North Atlantic (sub)surface temperature and seawater δ18O reconstructions during the intensification of Northern Hemisphere glaciationSouthward shifts of Subarctic Front during the intensification of Northern Hemisphere glaciation not analogous to the Last Glacial MaximumDust and/or eddies are important drivers of glacial export productivity at 41°N from 2.7 Ma [ABSTRACT FROM AUTHOR]

Marine algae are instrumental in carbon cycling and atmospheric carbon dioxide (CO2) regulation. One group, coccolithophores, uses carbon to photosynthesize and to calcify, covering their cells with chalk platelets (coccoliths). How ocean acidification influences coccolithophore calcification is strongly debated, and the effects of carbonate chemistry changes in the geological past are poorly understood. This paper relates degree of coccolith calcification to cellular calcification, and presents the first records of size-normalized coccolith thickness spanning the last 14 Myr from tropical oceans. Degree of calcification was highest in the low-pH, high-CO2 Miocene ocean, but decreased significantly between 6 and 4 Myr ago. Based on this and concurrent trends in a new alkenone ɛp record, we propose that decreasing CO2 partly drove the observed trend via reduced cellular bicarbonate allocation to calcification. This trend reversed in the late Pleistocene despite low CO2, suggesting an additional regulator of calcification such as alkalinity.