Comment on cp-2023-14

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> 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&rsquo;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 <em>Reticulofenestra haqii</em> and <em>Reticulofenestra antarctica</em>, 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 &delta;¹⁵N_org. values (&gt; 6 &permil;) 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.