The present-day interchange of water and heat between the Atlantic, Pacific, and Indian Oceans has been described through observation (physical properties and float studies) and numerical modeling at surface and intermediate water depths. The three ocean basin gyres of the Southern Hemisphere are strongly coupled in a system known as the Southern Hemisphere Supergyre (SHS). The Tasman Leakage (TL) branch of the SHS drives surface-to-intermediate waters from the Pacific into the Indian Ocean through an oceanic corridor/deeper pathway south of Australia (Figure 1). The SHS has become commonplace in discussions on past, present, and future thermohaline circulation, but the TL branch remains understudied and its geologic history largely unknown. In particular, the TL expression in the sedimentary record has not been established.

<p>Earth’s climate during the Neogene period changed in several steps from a planet with unipolar ice sheets to today’s bipolar configuration. Yet, time-continuous and well-preserved sedimentary archives from this time interval are scarce. This is especially true for those records that can be used for tracing the role of astronomical climate forcing. Ocean Drilling Program (ODP) Site 752 was drilled on Broken Ridge (Indian Ocean) and provides a time-continuous sedimentation history since the early Miocene in its upper portion.  To date, no astronomical-scale paleoclimate research has been conducted on this legacy ODP site. Here, we use X-ray fluorescence (XRF) core scanning data and benthic foraminifera (BF) taxonomic and quantitative analyses to reconstruct the paleoceanographic changes in the Indian Ocean since 23 Ma. Productivity-related elements from the XRF dataset, show higher productivity during the early Miocene and late Pliocene/early Pleistocene. Moreover, we found strong 405-kyr and ~110-kyr eccentricity imprints in the spectral analysis result of this XRF-derived paleoproductivity proxy. Although the precession signal is also quite remarkable in the spectral analysis results, the 4-cm resolution may not be adequate to further test the precession contribution. Bottom water oxygenation reconstructed using BF, suggest no oxygen minimum zone conditions for the late Miocene on site 752. Dissolved oxygen concentrations (DOC) indicate low oxic conditions (⁓ 2 ml/L) during this time, and relatively low stress species distribution (< 32%) along with abundant oxic species like <em>H. boueana, C. mundulus, L. pauperata </em>and<em> Gyroidinoides</em> spp. suggest predominantly high oxic conditions during the late Miocene (DOC > 2 ml/L). Meanwhile, the grain size (> 425µm) record shows an increasing trend at ~5 Ma, which indicates more current winnowing. Therefore, we argue that the drop in Mn is the result of the increase in the current winnowing, instead of the OMZ expansion. On the other hand, high-amplitude changes in Fe content from the lower Miocene to the middle Miocene, cannot be explained by eolian input, suggesting the source might be the neighbor-distanced Amsterdam-St. Paul hot spot. The source of Fe might be the neighbor-distanced Amsterdam-St. Paul hot spot. We conclude that the legacy ODP Site 752 constitutes an excellent paleoceanographic archive that allows us to reconstruct Indian Ocean dynamics since the early Miocene. New drillings on Broken Ridge with state-of-the-art scientific ocean drilling techniques will provide more detailed information and be highly beneficial for paleoclimatic and paleoceanographic research.</p>

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<p>The Southern Hemisphere Supergyre refers to the strong connections and intertwining of the southern subtropical gyres. Tasman Leakage is a fundamental part of the supergyre, as well as of the  global thermohaline circulation, as it provides a return flow from the Pacific and Indian Oceans to the North Atlantic at intermediate depths.   However, both are only relatively recently documented, and the timing and conditions of onset are not well understood.</p> <p>This study characterizes the newly identified onset of Tasman Leakage in sedimentary records in and around the Indian Ocean using core descriptions and data derived from sediments.  Since much of this is legacy core material, core photographs were used to develop complementary and more continuous records to help refine the timing of onset.  These newly constructed time series based on core photographs are compared with X-ray Fluorescence time series based on core scanning provide both insight into onset of Tasman Leakage and a first test of the utility of time series based on core photos.</p> <p>This effort will focus on the intermediate water pathway associated with Tasman Leakage and identify conditions at critical around the basin from at least 8 Ma at Broken Ridge and Mascarene Plateau to understand the role of Indian Ocean intermediate waters in the Southern Hemisphere Supergyre in major climate events of the late Miocene. </p> <p>This proposed work provides the first synoptic view of SHS onset using intermediate depth cores, which in turn will provide an important framework for basin-wide synthesis of Indian Ocean drilling, much of which is outside of the main pathway of the SHS.  It will also serve as a test of the utility of legacy material as primary data.</p>

<p>Inter-basinal heat and water exchange play a prominent role in driving global climate change on astronomical timescales, as part of the global thermohaline circulation. Tasman Leakage connects the Pacific and Indian Oceans at an intermediate water depth, south of Australia. Therewith, Tasman Leakage advects heat toward the Indian Ocean, and ultimately toward the Agulhas system. Hence, Tasman Leakage constitutes a non-negligible part of the present-day thermohaline circulation. The onset of Tasman Leakage likely occurred sometime in the Late Miocene (Christensen et al., 2021), but precise geochronology for the establishment of this inter-basinal connection is still lacking. Moreover, Tasman Leakage sensitivity to astronomical forcing remains to be constrained in detail. To understand Tasman Leakage on astronomical timescales, we present a new Miocene-to-recent multi-proxy dataset from Ocean Drilling Program (ODP) Sites 752 and 754, cored on Broken Ridge (30°53.475’S), southeastern Indian Ocean.</p><p>The dataset consists of new X-ray Fluorescence (XRF) core scans that provide element contents for 18 different elements, along with benthic carbon and oxygen stable isotopic records at 4 cm resolution. The XRF-derived Ca/Fe record is paced by 405-kyr eccentricity between 22 Ma and 13 Ma (early-middle Miocene), but then becomes more sensitive to obliquity and precession forcing. The new high-resolution benthic δ<sup>13</sup>C record confirms the onset of Tasman Leakage in the Late Miocene, more specifically at 6.6 Ma. This is when the Broken Ridge benthic δ<sup>13</sup>C signature no longer reflects an Antarctic Intermediate Water signal. The benthic δ<sup>18</sup>O record shows a strong ~110-kyr eccentricity imprint, indicating that Tasman Leakage might be most sensitive to this astronomical parameter. We conclude that the Neogene nannofossil oozes, preserved on Broken Ridge, constitute an excellent paleoceanographic archive that allows us to fingerprint Tasman Leakage sensitivity to astronomical forcing.</p>