Maastrichtian–Rupelian paleoclimates in the southwest Pacific – a critical re-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. Here, we use the current proxy insights to (re-)assess the reliability of the isoGDGT-based SST signal in 69 newly analyzed and 242 reanalyzed sediments at Ocean Drilling Program (ODP) Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques. We compare our results with paleoenvironmental and paleoclimatologic reconstructions based on dinoflagellate cysts. The resulting ∼ 130 kyr resolution Maastrichtian–Oligocene SST record based on the TetraEther indeX of tetraethers with 86 carbon atoms (TEX86) 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 Methylation index of Branched Tetraethers with 5-methyl bonds (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 subsidence 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. 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 that most of the generated data are reliable. However, this also implies that 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 without definitive explanation.

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

“…The vegetation-based summer temperature reconstructions of ca. 18.5 • C closely corroborate the brGDGT-biomarker reconstructions from the same site (Bijl et al, 2021), supporting the notion of a potential seasonal bias of this palaeothermometer (Contreras et al, 2014;Naafs et al, 2017). The warmth-loving taxa formed the main lowland forest components occupying sheltered areas and lowland subtropical coastal zones (Dowe, 2010;Car-penter et al, 2012;Tripathi and Srivastava, 2012;Verma et al, 2020) and swamps (Kershaw, 1988).…”

“…A dredging exercise confirmed the continental origin 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 from being the potential source of the terrestrial organic matter (Bijl et al, 2021). In addition, common Permo-Triassic reworked elements in our late Eocene-early Oligocene sporomorph assemblage likely indicate an eastern Tasmanian sporomorph source, in line with the Permian-Triassic upper Parmeener Group that contains terrestrial deposits and presently makes up the surface lithology across east Tasmania.…”

“…Holes A and D of ODP Site 1172 on the East Tasman Plateau both yielded EOT records and have been analysed for their pollen and spore contents. The age model relies on magnetostratigraphy (which has a particularly clear signal in the late Eocene; Stickley et al, 2004;Fuller and Touchard, 2004) and biostratigraphy (dinoflagellate cysts, nannoplankton, and diatoms; Stickley et al, 2004;Bijl et al, 2013), as presented in Houben et al (2019) and Bijl et al (2021).…”

“…To test the validity of our NLR-based climate estimates, we compare them to previously published independent botanical or geochemical proxies in the southern high latitudes spanning the late Eocene to early Oligocene (e.g. Colwyn and Hren, 2019;Houben et al, 2019;Korasidis et al, 2019;Bijl et al, 2021;Lauretano et al, 2021;Tibbett et al, 2021). Overall, these are generally in agreement and provide a certain level of confidence in the utility of the NLR-based climate estimates.…”

“…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 compare our pollen-based quantitative climate estimates with newly published TEX 86based sea-surface temperature (SST) and mean annual air temperature (MAAT soil ) reconstructions from the same site (Bijl et al, 2021). Our study reveals a significant terrestrial cooling ∼ 3 Myr prior to the EOT, and a warming in the earliest Oligocene that is most likely controlled by transient rebound of atmospheric pCO 2 and sustained deepening of the Tasmanian Gateway.…”