Surface-circulation change in the Southern Ocean across the Middle Eocene Climatic Optimum: inferences from dinoflagellate cysts and biomarker paleothermometry

Abstract

Abstract. Global climate cooled from the early Eocene hothouse (~ 52–50 Ma) to the latest Eocene (~ 34 Ma). At the same time, the tectonic evolution of the Southern Ocean was characterized by the opening and deepening of circum-Antarctic gateways, which affected both surface- and deep-ocean circulation. The Tasman Gateway played a key role in regulating ocean throughflow between Australia and Antarctica. Southern Ocean surface currents through and around the Tasman Gateway have left recognizable tracers in the spatiotemporal distribution of plankton fossils, including organic-walled dinoflagellate cysts. This spatiotemporal distribution depends on physico-chemical properties of the water masses in which these organisms thrived. The degree to which the geographic path of surface currents (primarily controlled by tectonism) or their physico-chemical properties (significantly impacted by climate) have controlled the composition of the fossil assemblages has, however, remained unclear. In fact, it is yet poorly understood to what extent oceanographic response as a whole was dictated by climate change, independent of tectonics-induced oceanographic changes that operate on longer time scales. To disentangle the effects of tectonism and climate in the southwest Pacific Ocean, we target a climatic deviation from the long-term Eocene cooling trend, a 500 thousand year long global warming phase termed the Middle Eocene Climatic Optimum (MECO; ~ 40 Ma). The MECO warming is unrelated to regional tectonism, and thus provides a test case to investigate the oceans physiochemical response to climate change only. We reconstruct changes in surface-water circulation and temperature in and around the Tasman Gateway during the MECO through new palynological and organic geochemical records from the central Tasman Gateway (Ocean Drilling Program Site 1170), the Otway Basin (southeastern Australia) and the Hampden Section (New Zealand). Our results confirm that dinocyst communities track tectonically driven circulation patterns, yet the variability within these communities can be driven by superimposed temperature change. Together with published results from the east of the Tasman Gateway, our results suggest that as surface-ocean temperatures rose, the East Australian Current extended further southward during the peak of MECO warmth. Simultaneous with high sea-surface temperatures in the Tasman Gateway area, pollen assemblages indicate warm temperate rainforests with paratropical elements along the southeastern margin of Australia. Finally, based on new age constraints we suggest that a regional southeast Australian transgression might have been caused by sea-level rise during MECO.

Maastrichtian-Rupelian paleoclimates in the southwest Pacific – a critical evaluation of biomarker paleothermometry and dinoflagellate cyst paleoecology at Ocean Drilling Program Site 1172

“…Post-MECO cooling seems more gradual than MECO warming, although this might be the result of sedimentation rate changes (Bijl et al, 2010), which are not accounted for in our age model. SSTs are almost identical to those for the MECO at nearby Site 1170 on the South Tasman Rise (Cramwinckel et al, 2020 (Houben et al, 2019).…”

“…Palynological evidence confirms that the Proto-Ross Gyre influence persisted at the ETP until the late Eocene (Bijl et al, 2011;Warnaar et al, 2009). This means that despite northward tectonic drift, the same strong western boundary current bathed the site during the Maastrichtian to early Eocene (Sijp et al, 2016), with perhaps some intermittent influence of East Australian Current waters from the north (Bijl et al, 2010;Cramwinckel et al, 2020). This ended when the proto-Leeuwin Current started to flow through the progressively widening TG ( Fig.…”

Abyssal oceanic circulation and acidification during the Middle Eocene Climatic Optimum (MECO)

“…The changes described represent the response of the complex oceanographic circulation of the SO to the MECO climatic warming 4,38 . Major circulation changes have been described in previous studies associated with the MECO climatic warming, at surface to bathyal depths 4,10,11,26,39,40 . Here we show that modifications occurred also in the abyssal circulation of the SO, with the inflow of more acid bottom water.…”

Campanian-Eocene dinoflagellate cyst biostratigraphy in the Southern Andean foreland basin: Implications for Drake Passage throughflow

“…ODP Site 1172 (Brinkhuis et al, 2003b;Bijl et al, 2013b;Houben et al, 2019); 4. Waipara, Tawanui, Ashley Mudstone and Hampden Beach (Crouch and Brinkhuis, 2005;Hollis et al, 2014;Crouch et al, 2014;Dallanave et al, 2016;Cramwinckel et al, 2019); 5. Seymour Island (Wrenn and Hart, 1988;Douglas et al, 2014;Aménabar et al, 2019); 6.…”

“…IODP Site U1356 (Bijl et al, 2013b); 8. ODP Site 1170 (Cramwinckel et al, 2019); 9. ODP Site 1168 (Brinkhuis et al, 2003a;Sluijs et al, 2003); 10.…”