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.
Introduction to the special issue on Volcanism in Zealandia and the SW Pacific
