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

Abstract

Abstract. Sea surface temperature (SST) reconstructions based on isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) distributions from the Eocene southwest (sw) Pacific Ocean are unequivocally warmer than can be reconciled with state-of-the-art fully coupled climate models. However, the SST signal preserved in sedimentary archives can be affected by contributions of additional isoGDGT sources. Methods now exist to identify and possibly correct for overprinting effects on the isoGDGT distribution in marine sediments. We here use the current proxy insights to assess the reliability of the isoGDGT-based SST signal in 69 newly analysed and 242 re-analysed sediments ODP Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques, in context of paleo-environmental and paleoclimatologic reconstructions based on dinoflagellate cysts. The resulting ~130 kyr-resolution Maastrichtian-Oligocene TEX86-based SST record confirms previous conclusions of anomalous warmth in the early Eocene sw Pacific and remarkably cool conditions during the mid-Paleocene. Dinocyst diversity and assemblages show a strong response to the local SST evolution, supporting the robustness of the TEX86 record. Soil-derived branched GDGTs stored in the same sediments are used to reconstruct mean annual air temperature (MAAT) of the nearby land using the MBT'5me proxy. MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen. General trends in SST and MAAT are similar, except for: 1) an enigmatic absence of MAAT rise during the Paleocene-Eocene Thermal Maximum and Middle Eocene Climatic Optimum, and 2) a subdued middle–late Eocene MAAT cooling relative to SST. Both dinocysts and GDGT signals suggest a mid-shelf depositional environment with strong river-runoff during the Paleocene-early Eocene, progressively becoming more marine thereafter. This trend reflects gradual drying and more pronounced wet/dry seasons in the northward drifting Australian hinterland, which may also explain the subdued middle Eocene MAAT cooling relative to that of SST. The overall correlation between dinocyst assemblages, marine biodiversity and SST changes suggests that temperature exerted a strong influence on the surface-water ecosystem, probably in part through sea level changes caused by steric effects. Finally, we find support for a potential temperature control on compositional changes of branched glycerol monoalkyl glycerol tetraethers (brGMGTs) in marine sediments. It is encouraging that a critical evaluation of the GDGT signals confirms the vast majority of the generated data is reliable. However, this also implies the high TEX86-based SSTs for the Eocene sw Pacific, and the systematic offset between absolute TEX86-based SST and MBT'5me-based MAAT estimates remain unexplained.

Late Paleocene CO<sub>2</sub> drawdown, climatic cooling, and terrestrial denudation in the southwest Pacific

“…Foraminiferal assemblages in the Taylor White succession confirm that this transgression progresses through the Waipawa Formation and into overlying Wanstead Formation (Naeher et al, 2019). A similar geological history is inferred for the Tasmanian margin where a major runoff event is linked to Waipawa deposition (Hill and Exon, 2004;Bijl et al, 2021). Therefore, the only plausible 575 explanation for a rapid influx of terrestrial plant matter is one or more eustatic falls in sea level, eroding coastal vegetation and flushing the debris into offshore basins.…”

“…We interpret this to indicate that Waipawa organofacies is the result of a rapid influx of terrestrial OM into the marine environment. Corroborating evidence for a marked increase in terrestrial runoff at this time has recently been reported in a 480 study of biomarkers and dinoflagellate paleoecology from ODP Site 1172 (Bijl et al 2021). The foraminiferal data (Naeher et al, 2019) suggest that this runoff event occurred while basins were progressively deepening, which is consistent with long-term passive-margin subsidence throughout New Zealand (King et al, 1999).…”

“…Positive correlations between δ 13 COM and the TEX86 SST proxy at mid-Waipara River and ODP Site 1172 (Fig. S8) indicate that Waipawa deposition is associated with significant cooling of coastal waters (Hollis et al, 2014;Bijl et al, 2021).…”

Eocene to Oligocene vegetation and climate in the Tasmanian Gateway region controlled by changes in ocean currents and pCO<sub>2</sub>

“…Dredging exercise confirms the continental origin of the of the plateau (Exon et al, 1997). However, the age of the guyot/seamount (dated as 36 Ma; Lanyon et al, 1993) disqualifies the ETP itself as the potential source of the terrestrial organic matter (Bijl et al, 2021).…”

“…The proximity of our study site to the Tasmanian Gateway places it in an excellent geographical position to identify potential climate or tectonic impacts on terrestrial vegetation of the Australo-Antarctica region. To further investigate potential links between the terrestrial and marine realm we also and mean annual air temperature (MAATsoil) reconstruction from the same site (Bijl et al, 2021). Our study reveals a significant terrestrial cooling ~3 Ma prior to the EOT, and a warming in the earliest Oligocene which is most likely controlled by transient rebound of atmospheric pCO2 and sustained deepening of the Tasmanian Gateway.…”