<p>Today, the western Arabian Sea represents one of the most productive marine areas in the world. The high productivity in this region is governed by upwelling related to the intensity of the South Asian Monsoon (SAM). Previous studies show that high productivity has prevailed since the late Early Miocene (~15 Ma) after establishing a favorable tectonic configuration in the region. Existing productivity records have further demonstrated that upwelling intensity varied in the western Arabian Sea over different time scales. This variability has been attributed mainly to changing monsoonal upwelling intensity linked to global climatic changes. However, the abundance and contribution of individual primary producers (calcareous nannoplankton and diatoms) have never been studied in the context of upwelling and SAM changes. To fully disentangle the variability in the context of local upwelling changes and nutrient availability at ODP Site 722B, we link assemblage-based primary productivity records to the established multi-proxy framework in the region. Quantitative nannofossil assemblage records and absolute diatom abundances are examined in conjunction with existing and new planktonic foraminifer data to better constrain the temporal variation in productivity in the western Arabian Sea.</p> <p>In our record, the first increase in cool and eutrophic nannofossil taxa (i.e., <em>Coccolithus pelagicus</em> and <em>Reticulofenestra pseudoumbilicus</em>) corresponds to the initial phase of sea surface temperatures (SST) cooling ~13.4 Ma. By ~12 Ma, rare occurrences of diatoms frustules correspond to the maximum abundances of <em>Reticulofenestra haqii</em> and <em>Reticulofenestra antarctica</em>, indicating higher upwelling derived nutrient levels. However, these changes ~12 Ma occur in the absence of coeval high latitude cooling, as shown by deep-sea benthic oxygen isotope records. By 11 Ma, diatom abundance increases significantly, leading to alternating blooms of upwelling sensitive diatom species (<em>Thalassionema </em>spp.) and eutrophic nannoplankton species (e.g., <em>R. pseudoumbilicus</em>). These changes in primary producers are also well reflected in geochemical proxies with increasing δ<sup>15</sup>N<sub>org.</sub> values (> 6‰) and high C/N ratios also confirming high productivity and beginning denitrification at the same time.</p> <p>Our multi-proxy-based evaluation of Site 722B primary producers thus indicates a stepwise evolution of productivity in the western Arabian Sea related to the intensity of upwelling and forcing SAM dynamics throughout the Middle to Late Miocene. The absence of full correspondence with existing deep marine climate records also suggests that local processes, such as lateral nutrient transport, likely played an important role in modulating productivity in the western Arabian Sea. We show that using a multi-proxy record provides novel insights into how fossil primary producers responded to changing nutrient conditions through time in a monsoon-wind-driven upwelling zone.</p>

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Abstract. Understanding the behavior of past upwelling cells is paramount when assessing future climate changes. Our present understanding of nutrient fluxes throughout the world's oceans emphasizes the importance of intermediate waters transporting nutrients from the Antarctic divergence into the middle and lower latitudes. These nutrient-rich waters fuel productivity within wind-driven upwelling cells in all major oceans. One such upwelling cell is located along the Oman Margin in the Western Arabian Sea (WAS). Driven by cross-hemispheral winds, the WAS upwelling zone’s intense productivity led to the formation of one of the most extensive oxygen minimum zones known today. In this study covering the Middle to Late Miocene at ODP Site 722, we investigate the inception of upwelling-derived primary productivity. We combine novel data with existing model- and data-based evidence, constraining the tectonic and atmospheric boundary conditions for an upwelling cell to exist in the region. With this research, we build upon the original planktonic foraminifer-based research by Dick Kroon in 1991 as part of his research based on the Ocean Drilling Project (ODP) LEG 117. We show that monsoonal winds likely sustained upwelling since the emergence of the Arabian Peninsula after the Miocene Climatic Optimum (MCO) ~14 Ma, with fully monsoonal conditions occurring since the end of the Middle Miocene Climatic Transition (MMCT) ~13 Ma. However, changing nutrient fluxes through Antarctic Intermediate and sub-Antarctic Mode Waters (AAIW/SAMW) were only established by ~12 Ma. Rare occurrences of diatoms frustules correspond to the maximum abundances of Reticulofenestra haqii and Reticulofenestra antarctica, indicating higher upwelling-derived nutrient levels. By 11 Ma, diatom abundance increases significantly, leading to alternating diatom blooms and high-nutrient-adapted nannoplankton taxa. These changes in primary producers are also well reflected in geochemical proxies with increasing δ15Norg. values (> 6 ‰) and high organic carbon accumulation also confirm high productivity and beginning denitrification simultaneously. Our multi-proxy-based evaluation of Site 722B primary producers thus indicates a stepwise evolution of productivity in the western Arabian Sea related to the intensity of upwelling and forcing SAM dynamics throughout the Middle to Late Miocene. The absence of full correspondence with existing deep marine climate records also suggests that local processes, such as lateral nutrient transport, likely played an important role in modulating productivity in the western Arabian Sea. Finally, we show that using a multi-proxy record provides novel insights into how fossil plankton responded to changing nutrient conditions through time in a monsoon-wind-driven upwelling zone.

<p>A recent biostratigraphic re-evaluation of Ocean Drilling Program (ODP) Site 722 (Bialik et al., accepted, Paleoceanogr. and Paleocl.) provides new insights into the history of monsoon driven upwelling in the Arabian Sea between 15 and 8.5 Ma. They suggest the modern monsoon was only established after tectonic preconditioning, linked to the uplift of the Himalayas, closure of the Tethyan Seaway, and the inception of Indonesian Throughflow restriction. But the requisite topography for the Indian monsoon was already in place by at least the late early Miocene which suggests another driver. However, as northern hemisphere latitudinal heat gradients continued to be shallower than modern throughout the Miocene, steepening southern hemisphere gradients during the middle Miocene glaciation of Antarctica ~14.8 Ma (Pound et al., 2012, Earth-Sci. Rev., 112) may have played an important role in pacing the monsoon system during the middle to late Miocene.</p><p>Here we further explore these findings by using recently acquired X-ray fluorescence (XRF) core scanning data from two additional ODP sites located in the central (Site 707) and southern (Site 752) Indian Ocean. We trace the timing and pacing of these environmental changes along a cross hemispheric transect within key areas of the larger Indian Ocean-Atmospheric system: (1) the monsoonal upwelling regions along the Oman Margin (Site 722); (2) the Somali/Findlater jets (Site 707); and (3) the high-pressure zone in the southern horse latitudes (Site 752).</p><p>Using updated age constraints at all sites, we show that the intensification of upwelling at Site 722 is tightly linked to climatic and oceanographic changes in the southern high latitudes (e.g., Groeneveld et al., 2017; Sci. Adv.). This close co-evolution of southern hemisphere climatic shifts and monsoon dynamics hints at a strong contribution of increasing southern hemisphere thermal gradients on the middle to late Miocene evolution of the Indian Ocean circulation system and Indian monsoon dynamics. Our findings thus re-emphasize the Indian summer monsoon as the result of a complex cross-hemispheric ocean-atmospheric system spanning the Indo-Pacific (e.g., Gadgil, 2018, J. Earth Syst. Sci., 127). We postulate that the Indian Ocean-Atmospheric system experienced a gradual intensification that began after the Middle Miocene Climatic Optimum with Antarctic Ice Sheet expansion. These changes then culminated in a synchronous shift ~11 Ma during the Ser4/Tor1 sea level lowstand (Haq et al., 1987; Science, 235). Future chrono-, chemo- and cyclostratigraphic work at ODP Sites 707 and 752 will further help to constrain the timing of these events, and fully place them in the context of the global climatic evolution during the Miocene.</p>