Latitudinal Migrations of the Subtropical Front at the Agulhas Plateau Through the Mid‐Pleistocene Transition

Abstract

The meridional variability of the Subtropical Front (STF) in the Southern Hemisphere, linked to expansions or contractions of the Southern Ocean, may have played an important role in global ocean circulation by moderating the magnitude of water exchange at the Indian-Atlantic Ocean Gateway, so called Agulhas Leakage. Here we present new biomarker records of upper water column temperature ( K' 37 U and 86 TEX ) and primary productivity (chlorins and alkenones) from marine sediments at IODP Site U1475 on the Agulhas Plateau, near the STF and within the Agulhas retroflection pathway. We use these multiproxy time-series records from 1.4 to 0.3 Ma to examine implied changes in the upper oceanographic conditions at the mid-Pleistocene transition (MPT, ca. 1.2-0.8 Ma). Our reconstructions, combined with prior evidence of migrations of the STF over the last 350 ka, suggest that in the Southwestern Indian Ocean the STF may have been further south from the Agulhas Plateau during the mid-Pleistocene Interim State (MPIS, MIS 23-12) and reached its northernmost position during MIS 34-24 and MIS 10. Comparison to a Globorotalia menardii-derived Agulhas Leakage reconstruction from the Cape Basin suggests that only the most extreme northward migrations of the STF are associated with reduced Agulhas Leakage. During the MPIS, STF migrations do not appear to control Agulhas Leakage variability, we suggest previously modeled shifting westerly winds may be responsible for the patterns observed. A detachment between STF migrations and Agulhas Leakage, in addition to invoking shifting westerly winds may also help explain changes in CO 2 ventilation seen during the MPIS.CARTAGENA-SIERRA ET AL.

Genome‐wide phylogeography reveals cryptic speciation in the circumglobal planktonic calcifier Limacina bulimoides

“…Changes in ocean circulation during the mid‐Pleistocene transition could have facilitated the physical separation and subsequent divergence of the three lineages across the various ocean basins. These changes include the reduced exchange between the Indian and Atlantic Oceans via the Agulhas leakage due to the northward migration of the Subtropical Front towards the Agulhas Plateau (Caley et al, 2012; Cartagena‐Sierra et al, 2021), and the connection between the Pacific and Indian Ocean due to the weakening of the Indonesian Throughflow (Petrick et al, 2019). Similar geographical structuring across ocean basins for other circumglobal warm‐water plankton species has been attributed to both physical (e.g., ocean currents or continental landmasses) and ecological (species‐specific interactions with oceanographic gradients) barriers (e.g., Bendif et al, 2019; Burridge et al, 2019; Filatov et al, 2021; Goetze et al, 2015; Hirai et al, 2015), although it is unknown if these structured populations arose at the same time.…”

The Sediment Green‐Blue Color Ratio as a Proxy for Biogenic Silica Productivity Along the Chilean Margin

“…Nonetheless, most degradation products of chlorophyll retain their original color (Reuss et al., 2005). The color properties of chlorins (a typical chlorophyll degradation product) are measurable by spectrophotometry method in Pleistocene sediments at various sedimentation settings (e.g., Cartagena‐Sierra et al., 2021; Gebhardt et al., 2008; Harris et al., 1996; Hodgson et al., 2003), reaching timescales over million years and burial depths of hundreds of meters (Cartagena‐Sierra et al., 2021). It is likely the G/B proxy is a more robust indicator and possibly independent of organic matter preservation.…”

Latitudinal Variance in the Drivers and Pacing of Warmth During Mid‐Pleistocene MIS 31 in the Antarctic Zone of the Southern Ocean

“…Sea‐ice is the likely mechanism responsible for this decoupling. Site 1,090 would have been closer to the productive sea ice edge during a glacial than during an interglacial, when the sea ice edge and associated nutrients would have been much farther south (Cartagena‐Sierra et al., 2021). Therefore, at this northern site favorable growth conditions for diatoms would have been present during glacials.…”

“…(2009) presented calcareous nannofossil evidence of warming greater than that seen during other interglacials over MIS‐31 at ODP Site 1,090. The position of the polar front was north of Site 1,090 during MIS‐31, further indicating warmth (Cartagena‐Sierra et al., 2021). EAIS retreat, at least in the vicinity of Prydz Bay (ODP Site 1,165), is supported by nannofossil evidence (Villa et al., 2008), however, continued iceberg‐rafting indicates that the EAIS and its marine‐terminating glaciers remained capable of shedding icebergs through MIS‐31 (Teitler et al., 2015).…”