Pliocene variations in the position of the Antarctic convergence in the southwest Atlantic

Abstract

Middle to late Pliocene (4.1–1.9 Ma) variations in the positions of surface water masses and migrations of the Polar Front in the southwest Atlantic are inferred from a factor analysis of radiolarian assemblages from DSDP site 514. Faunal results suggest that surface water masses underwent progressive cooling as the Polar Front Zone (PFZ) advanced northward during the latest Gilbert to late Gauss chron. The late Gilbert chron (4.12–3.86 Ma) was the waning phase of a mid‐Gilbert warm interval. Northern Subantarctic surface water prevailed between 4.12 and 3.97 Ma, indicative of surface water temperatures warmer than those during the succeeding Pliocene. A long‐term sea surface cooling trend began between 3.97 and 3.86 Ma which moved the PFZ northward and closer to site 514, bringing cooler southern Subantarctic surface waters to the site. This movement northward of cooler surface waters is inferred to be related to the initial growth of sea ice and ice shelves throughout regions of West Antarctica which were deglaciated or unglaciated during the prior warming interval of the Gilbert chron. A major late Gilbert/early Gauss chron increase in the production of Antarctic Bottom Water (AABW) resulted in an erosional or nondepositional hiatus at site 514, spanning 3.86 to 3.18 Ma. Increased AABW activity at this time is linked to continued growth of expansive ice shelves and sea ice in the West Antarctic region, in the East Antarctic margin, and possibly in the marine embayments of East Antarctica. Decreased AABW production led to a 3.2 Ma resumption of sedimentation at site 514 and elsewhere in the southern ocean. Antarctic surface water assemblages above the hiatus (3.18–3.16 Ma) suggest that the PFZ first passed over site 514 during the 3.86–3.18 Ma interval of the hiatus. Following this initial invasion of Antarctic surface waters, between 3.16 and 2.94 Ma the PFZ retreated to a position near or south of the site. This slight retreat of the PFZ, with a corresponding decrease in AABW activity, is possibly related to a significant reduction in the area of shelf and sea ice as grounded ice expanded in the West Antarctic region. We interpret the widespread global enrichment of δ18O at approximately 3.2 Ma to be partially related to a cooling of surface waters, the production of cooler bottom waters, and an expansion of West Antarctic ice volume associated with its grounding phase between 3.5 and 2.7 Ma. Based on surface water mass migrations at site 514, the West Antarctic ice volume increased significantly slightly prior to or at approximately 3.2 Ma, decreased between 3.14 and 2.7 Ma, and then increased by 2.67 Ma to a more stable configuration with a volume at least as great as that at 3.2 Ma. During the late Gauss (2.94–2.47 Ma), four northward advances of the PFZ led to the migration of Antarctic surface waters over the site by 2.67 Ma and continued dominance of Antarctic surface water through the remainder of the Pliocene. We suggest that this permanent change from Subantarctic to Antarctic surface water mass dominance was linked indirectly to the initial growth of northern hemisphere ice and a reduction of sea level. We propose a model of simultaneous northern and southern hemisphere ice volume growth induced by interhemisphere positive feedback which is responsible for the major late Gauss advance of Antarctic surface waters and the global enrichment of δ18O at 2.5 Ma.