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

Abstract

Abstract. Considered as one of the most significant climate reorganisations of the Cenozoic period, the Eocene-Oligocene Transition (EOT; ca. 34.44–33.65) is characterised by global cooling and the first major glacial advance on Antarctica. While in the southern high-latitudes, the EOT cooling is primarily recorded in the marine realm, the extent and effect on terrestrial climate and vegetation is poorly documented. Here, we present a new, well-dated, continuous, high-resolution palynological (sporomorph) data and quantitative sporomorph-based climate estimates recovered from the East Tasman Plateau (ODP Site 1172) to reconstruct climate and vegetation dynamics from the late Eocene (37.97 Ma) to early Oligocene (33.06 Ma). Our results indicate three major climate transitions and four vegetation communities occupying Tasmania under different precipitation and temperature regimes: (i) a warm-temperate Nothofagus-Podocarpaceae dominated rainforest with paratropical elements from 37.97–37.52 Ma; (ii) cool-temperate Nothofagus dominated rainforest with secondary Podocarpaceae rapidly expanding and taking over regions previously occupied by the warmer taxa between 37.306–35.60 Ma; (iii) fluctuation between warm temperate – paratropical taxa and cool temperate forest from 35.50–34.49 Ma, followed by a cool phase across the EOT (34.30–33.82 Ma); (iv) post-EOT (earliest Oligocene) recovery characterised by a warm-temperate forest association from 33.55–33.06 Ma. Coincident with changes in stratification of water masses and sequestration of carbon from surface water in the Southern Ocean, our sporomorph-based temperature estimates between 37.52 Ma and 35.60 Ma (phase ii) showed 2–3 °C terrestrial cooling. The unusual fluctuation between warm and cold temperate forest between 35.50 to 34.59 Ma is suggested to be linked to the initial deepening of the Tasmanian Gateway allowing eastern Tasmania to come under the influence of warm water associated with the proto-Leeuwin Current (PLC). Further to the above, our terrestrial data show mean annual temperature declining by about 2 °C across the EOT before recovering in the earliest Oligocene. This phenomenon is synchronous with regional and global cooling during the EOT and linked to declining pCO2. However, the earliest Oligocene climate rebound along eastern Tasmania is linked to transient recovery of atmospheric pCO2 and sustained deepening of the Tasmanian Gateway, promoting PLC throughflow. The three main climate transitional events across the studied interval (late Eocene–earliest Oligocene) in the Tasmanian Gateway region suggest that changes in ocean circulation due to accelerated deepening of the Tasmanian Gateway may not have been solely responsible for the changes in terrestrial climate and vegetation dynamics, but a series of regional and global events, including a change in stratification of water masses, sequestration of carbon from surface waters, and changes in pCO2 may have played vital roles.

Proxy‐Model Comparison for the Eocene‐Oligocene Transition in Southern High Latitudes

“…The NLR approach assumes that the climate tolerances of past species are similar to their modern relatives (Hollis et al., 2019) and uncertainty depends on the quality of modern data and identification of fossil taxa (Utescher et al., 2014). As usual, the best way to corroborate such uncertainties is through cross checks with independent data such as that available from leaf traits (Pound & Salzmann, 2017) and temperature reconstructions included here (Lauretano et al., 2021; Tibbett, Scher, Warny, Tierney, Passchier, et al., 2021) that support the NLR results (Amoo et al., 2022; Thompson et al., 2022).…”

“…For clumped isotopes uncertainties come from instrument error, sample heterogeneity and accuracy summarized as 2.5°C in the Seymour Island study (Douglas et al., 2014). Pollen temperatures generated from NLR analysis are reported to have a standard deviation of 2°C–3°C (Amoo et al., 2022; Thompson et al., 2022). For S‐index the reported calibration standard deviation is 3.6°C (Sheldon et al., 2002).…”

“…(a) Map depicting location of records used in this study. (b) Land surface mean air temperature (MAT) data including BayMBT (Tibbett, Scher, Warny, Tierney, Passchier, et al., 2021; Tibbett et al., 2022c), pollen climate reconstructions including nearest living relative (Amoo et al., 2022; Francis et al., 2008; Macphail & Truswell, 2004; Passchier et al., 2013; Poole et al., 2005; Thompson et al., 2022; Truswell & Macphail, 2009), S‐index (Passchier et al., 2013, 2017), and MBT/Cyclization of Branched Tetraethers (Douglas et al., 2014). (c) Sea surface temperature (SST) data from BAYSPAR calibrated TEX 86 data (Douglas et al., 2014; Hartman et al., 2018; Houben et al., 2019; Lauretano et al., 2021; Pagani et al., 2011; Tibbett, Scher, Warny, Tierney, Passchier, et al., 2021), SSTs from BAYSPLINE calibrated U k’ 37 (Houben et al., 2019; Liu et al., 2009; Pagani et al., 2011; Plancq et al., 2014), and Δ 47 from bivalves (Douglas et al., 2014; Petersen & Schrag, 2015).…”

“…For the southern continents, reconstructions of mean annual air temperatures (MAT, Table 1) come from palynological analysis (Francis et al., 2008; Hunt & Poole, 2003; Macphail & Truswell, 2004; Poole et al., 2005; Truswell & Macphail, 2009), which identifies pollen grains to plant genus or species level and constrains the climate based on known temperature and precipitation ranges of the extant species or nearest living relative (NLR) (Amoo et al., 2022; Thompson et al., 2022), as a probability density function (and central estimate) of likely climatic range (Harbert & Nixon, 2015; Hollis et al., 2019; Willard et al., 2019). The temperature compilation for the continents also includes mineral weathering via the S‐index climofunction (Passchier et al., 2013, 2017) which is based on the molar ratio of Na 2 O and K 2 O to Al 2 O released during weathering (Sheldon et al., 2002).…”

Late Eocene–early Miocene evolution of the southern Australian subtropical front: a marine palynological approach

“…We also included a broad count of terrestrial palynomorphs in order to calculate the relative abundances between terrestrial and marine palynomorphs. Detailed analyses of terrestrial palynomorphs and terrestrial palaeoenvironmental evolution at Site 1168 is the focus of another paper (Amoo et al, 2021).…”