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.
Highlights
Before the TEX86 index results (Fig. 4a) can be interpreted in terms of Sea surface temperature (SST), we assess whether the isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) are primarily derived from the sea surface or have received contributions from non-pelagic sources
Given that the relatively low #ringstetra values argue against a large contribution from marine in situ production, we suggest that river-produced 6methyl Branched GDGTs (brGDGTs) have contributed to the GDGT pool in the Paleocene samples with high isomerization ratio (IR)
BrGDGTs at this site are likely primarily soil or peat derived; albeit evolutionary changes in brGDGT producers may be responsible for Paleogene-specific brGDGT signals
Summary
1.1 The Paleogene southwest Pacific OceanReconstructions of deep-sea (Westerhold et al, 2020) and sea surface temperature (Bijl et al, 2009, 2013a; Hollis et al, 2012, 2019; Frieling et al, 2014; Inglis et al, 2015; O’Brien et al, 2017; Evans et al, 2018; Cramwinckel et al, 2018; O’Connor et al, 2019; Sluijs et al, 2020) have revealed an overall cool climate in the Maastrichtian and Paleocene, long-term warming towards the early Eocene Climatic Optimum (EECO; 53.4–49.2 Ma) and subsequent cooling during the middle and late Eocene (48.6–33.6 Ma). The absolute SST estimates for the SW Pacific are closer to those from the equatorial regions than they are to the deep sea (Cramwinckel et al, 2018), which is implausible given that the South Pacific is inferred to have been the dominant region of deep-water formation during the Eocene (Huber and Thomas, 2010; Thomas et al, 2003, 2014) Without this model–data mismatch resolved, it remains unclear to what extent climate models properly simulate polar amplification or, alternatively, if the current proxies properly reflect highlatitude temperatures under greenhouse conditions (Lunt et al, 2012)
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