Deep circulation changes in the South Atlantic Ocean: Response to initiation of northern hemisphere glaciation

Abstract

Particle size statistics (coarsest 1 percentile and mean) of the nonbiogenic silt fraction of samples from Deep Sea Drilling Project sites 514, 516A, 517, 518, and 548 were used to infer relative changes in paleospeed of deep water masses in the Atlantic Ocean during the late Pliocene initiation of northern hemisphere glaciation. Grain size statistics were integrated with previous studies of geochemical and faunal assemblages to examine how climatic forcing regulates deepwater mass paleospeed and stratification. On the basis of between‐site comparison of particle size, the late Pliocene was divided into four intervals of deepwater activity: 3.2 to 2.9 Ma, 2.9 to 2.6 Ma, 2.6 to 2.5 Ma, and 2.5 to 2.3 Ma, with pulses of increased paleospeed occurring at 3.15–3.10, 2.85, 2.7, 2.6, and 2.4 Ma. Except for a brief increase in paleospeeds at 3.1 Ma, the first time interval is characterized by low inferred Antarctic Bottom Water (AABW) paleospeed corresponding to depleted δ13C and enriched δ18O values in all but the shallowest core. Enriched δ13C and depleted δ18O values in the shallowest core correspond to coarse grain sizes and are inferred to represent a deep and intense Antarctic Intermediate Water (AAIW). The expansion of AABW and AAIW precludes a significant North Atlantic Deep Water (NADW) incursion to 30°S at this time. The interval from 2.9 to 2.6 Ma is dominated by climatic cooling and an influx of deep waters originating in the North Atlantic. The introduction of significant volumes of Mediterranean Outflow Water (MOW), as demonstrated by increased inferred paleospeed at site 548, and an expanding glaciation beginning at 2.9 Ma, resulted in increased paleospeed and an expansion of NADW in the South Atlantic. A pulse of increased inferred paleospeed at 2.7 Ma resulted in the temporary deepening of NADW to the depth of site 518 (presently 3944 m). From 2.9 to 2.6 Ma AABW velocity remained low, but AAIW deepened and intensified. The 2.6‐to 2.5‐Ma interval represents the retreat of deep water, originating in the North Atlantic, from the depth of site 518 to the depth of site 517 (presently 2963 m). Increased inferred paleospeed at site 518 corresponds with depleted δ13C values and a northward shift in the polar front, thus suggesting the reintroduction of Circumpolar Water to the depth of site 518 and the establishment of a deepwater stratification similar to today's. A continuing trend toward a modern stratification continued into the latest time interval (2.5–2.3 Ma) as inferred deepwater paleospeed at all depths increased in response to the formation of extensive northern hemisphere ice sheets.