Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Shelf Sediments Reveal Shifts in Southern Hemisphere Westerlies in the Miocene

When the Antarctic ice sheet expanded about 14 million years ago (~14 Ma) there was a major reorganisation of the global climate (Zachos, Dickens & Zeebe, 2008). Global atmospheric circulation was affected by this event, which included the strength and position of the westerlies and the Intertropical Convergence Zone (ICZ) and, therefore, the patterns of precipitation (Ao et al., 2016; DeConto, Pollard & Harwood, 2007; Holbourn et al., 2010; Shevenell, Kennett& Lea, 2004). In this paper Groeneveld et al. present new shallow-marine sediment records from the Australian continental shelf (International Discovery Program Sites U1459 and U1464) which provide the first empirical evidence of high latitude cooling around Antarctica to climate change in the (sub)tropics during the Miocene. It was shown by this study that during most of the Miocene Western Australia was arid. During the Late Miocene southwestern Australia became wetter, which formed a climate gradient with the arid interior, though northwest Australia remained arid throughout. In southwest Australia precipitation and river runoff increased gradually from 12 to 8 Ma, which this study related to a northwards migration or intensification of the westerlies that was possibly the result of increased sea ice in the Southern Ocean (Shevenell, Kennett & Lea, 2004). The westerlies are indicated by abrupt aridification to have shifted back to a position south of Australia after 8 Ma. The midlatitude data from this study are consistent with the inference that the expansion of sea ice around Antarctica had the result of a northwards movement of the westerlies. This, in turn, may have pushed tropical atmospheric circulation and the ITCZ northwards, which shifted the mean precipitation belt over large parts of Southeast Asia (Holbourn et al., 2010).

Located between Australia and Antarctica, the midlatitude westerlies are part of the Southern Annular Mode (SAM) (Marshall, 2003). The zonal mean atmospheric pressure differences between the midlatitudes and Antarctica are defined by the SAM. A strengthening of the northward migration of the westerlies of the Southern Hemisphere results from this increase in atmospheric pressure contrast (Marshall, 2003; Raut, Jakob & 2014). The westerly, accordingly, bring precipitation to southwest Australia during the austral winter. Sea surface temperatures (SSTs) and the extent of sea ice around Antarctica also influence the position of the westerlies (Fan, Deser & Schneider, 2014). During the austral winter the increase in Antarctica sea ice cover pushes the westerlies, and consequently the Hadley cell northwards, which intensifies the atmospheric circulation over Australia (Ao et al., 2016; Schneider, Bischoff & Haug, 2014; Chiang & Bitz, 2005; Toggweiler & Lea, 2010). The Southern Hemisphere Hadley circulation connects the dry descending air of the subtropical ridge with low latitude moisture and air that is rising of the Intertropical Convergence Zone (ITCZ). The summer and winter monsoons over Southeast Asia and Australia, are controlled by the seasonal movement of the ITCZ. Changes in the SAM in recent decades have led to a drying in southwest Australia (Marshall, 2003), though it is still not clear how this climate response is expressed on longer, million-year timescales.

In Western Australia of the present the continental shelf is directly adjacent to the semiarid continent and hosts an archive of climate variability that has barely been explored. Recently sediments were recovered that during the International Ocean Discovery Program (IODP) Expedition 356 that provided new insights about the Miocene history of aridity and humidity in Australia (Gallagher et al., 2014). The Australian continent was located 5^o to 10^o further to the south than at the present (Müller et al., 2008), so that southwest Australia was under the direct influence of the westerlies. Southwest Australia would have been gradually shifted out of the influence of the westerlies and enhanced aridity by the northwards tectonic movement alone, assuming weather systems remained the same. It is shown by the data of the present study, however, that there was an intensification of precipitation and fluvial runoff during this time, which suggests the influence of additional controls.

In the Perth Basin IODP site U1459 and site U1464 in the Roebuck Basin are the southernmost and northernmost that were drilled in a latitudinal transect (Gallaher, Fulthorpe & Bogus, 2014). Ages of both sites from the earliest Middle Miocene [~16 -18 Ma (million years ago)] to the Late Miocene (5.59 Ma), are restrained by nannofossil and benthic foraminiferal biostratigraphy. An eccentric-controlled signal was documented by time series analysis of the downhole wireline logs, which supports the biostratigraphic age model, which suggests the sedimentation rates were relatively constant on million-year time scales. The sediments consists of shallow-marine, and, possibly subaerial deposits that include sediments and evaporitic nodules that are tidally laminated at the northern site U1464, which suggests arid conditions. Sabkha-like sediments that include layers that are organic-rich and carbonate-poor layers represent the most arid interval (~14.1 to 12.6 Ma). In contrast with this, sediments that were recovered at the southern site (U1459) are typified by grain-supported dolomitic limestones with a few intervals of sandstones that are rich in quartz deposited in the shelf setting (>100 m water depth) influenced by river input.

A continuous record of the degree of siliciclastic riverine input is provided by downhole wireline logs of potassium (K), thorium (Th) and their ratio Th/K at both sites that are carbonate dominated. The K component of the gamma ray logs is often used to infer the concentration of K-bearing aluminosilicates, mainly in the form of clays and K feldspars (Calvert & Pedersen, 2007). Previously, an increase in K was interpreted as a relative increase in the supply of siliciclastic sediment via rivers, which is indicative of a change from drier to wetter conditions off the coast of Western Australia (Kuhnt et al., 2015; Stuut, Temmesfeld & Deckker, 2014). It was confirmed that the K feldspars are the main minerals that bear K in the sediments of site U1459, by X-ray diffraction (XRD) analyses to determine bulk mineralogy. Thorium occurs as aluminosilicates as well as in heavy minerals, which are mainly transported by wind, and therefore are related to arid conditions. Therefore, in this study the Th/K ratio was used to provide a record that was independent of dilution, and this was supported by variations in the absolute K content. For the palaeodrainage systems of Australia the configuration during the Miocene was similar to those of the present (Morgan, 1993). Therefore, it was assumed that the pathways of sediment and their relationship to continental climate were also similar to conditions at the present. Accordingly, high Th/K values as being indicative of dry conditions and high K values showing variations in riverine input and, thus, the amount of rainfall over the southwest of Australia. The highest K accompanied by low K (%) occurred prior to 11 Ma with significant amplitude of variability. A switch to Th/K that was generally low, and increasing % of K occurred after 11 Ma. At site U1464 highest Th/K values show that the northwest was affected primarily by arid conditions, whereas throughout the studied time interval the K values remain low. It is indicated by these results that conditions that were influenced by a river were found mainly in southwest Australia. This contrasts with indications for wet conditions and rainforests during the Middle Miocene in central and Western Australia as well as in the southeast (Martin, 2006). On the other hand, the findings of this study agree with modelling studies, which suggest that the monsoon was no more intense than at present (Herold et al., 2011). Also, no univocal evidence has been found for humid vegetation types for the interior in the Middle Miocene (Mètzger & Retallack, 2010). After the slightly wetter conditions in southwest Australia during the Middle Miocene, conditions became as arid as in the northwest. This is indicated by Th/K values around 20 and very low K (%) (<0.01%) at Site U1459 from ~14 to 12 Ma, in close connection with the expansion of the Antarctic Ice Sheet (AIS). Th/K was consistently <10 and there was an increase in K values, from 12 to 8 Ma, which points to conditions that are progressively wetter in southwest Australia. Conditions returned to a drier setting after 8 Ma.

A framework to put in a global perspective is provided by high resolution records from this study. The Miocene Climatic Optimum (~17 to 15 Ma) (Zachos et al., 2008), the globally warm period, was truncated by a major increase in the volume of ice on Antarctica along with a decrease in global sea level at ~13.9 Ma, was possibly the result of a global decline in atmospheric CO₂ (Zachos, Dickens & Zeebe, 2008; Zhang et al., 2013), or by a switch to orbital forcing (Holbourn, Kuhnt, Schulz & Erlenkeuser, 1995). The ice sheet remained relatively stable until ~8 Ma, following the initial large increase the volume of the ice, though the temperatures of the deep sea continued decreasing (Zachos, Dickens & Zeebe, 2008; De Boer et al., 2012). Increasingly wet conditions continued in southwest Australia and this was accompanied by continued cooling of the sea surface of the Southern Ocean, reaching a maximum at ~8 Ma (Gersonde & Censarek, 2006). A connection between cooling water masses in the Southern Ocean, an increase in a cover of sea ice around Antarctica, and the northwards migration of the westerlies in the Late Miocene, was proposed by Groeneveld et al., (2017). A northwards migration of the Hadley cell may have also resulted from the northwards migration of the westerlies. Both an increase in the size of the AIS at ~13.9 Ma (DeConto, Pollard & Harwood, 2007), and continued global oceanic cooling well into the Late Miocene (De Boer et al., 2012; Herbert et al., 2016) would have been facilitated by the formation of sea ice around Antarctica. This supports the expansion of sea ice around Antarctica (Gersonde & Censarek, 2006), which led to widespread atmospheric change in the Southern Hemisphere.

It is argued by Groeneveld et al. that changing precipitation and river runoff reflect differences in the influence of the westerlies over southwest Australia. It is implied by this that the northward extent of the westerlies reached 30^oS to 40^oS from about 12 to 8 Ma. It was suggested by model experiments on the Miocene glaciation (Ao et al., 2016; DeConto, Pollard & Harwood, 2007) and the reconstructions of relative seawater salinity from the South China Sea that both the westerlies and the ITCZ moved northwards during the Late Miocene and intensified, though the timing and rate of change had remained unknown until this study. Decreasing temperatures and freshening from the Middle Miocene to more glacial conditions following the expansion of the AIS, that was interpreted as a northwards movement of the Subantarctic Front (Shevenell, Kennett & Lea, 2004; Kuhnert, Bickert & Paulsen, 2009), was shown by sea surface records from the Atlantic section of the Southern Ocean        [(Ocean Drilling Program (ODP) site U1092] and the Tasman Rise (ODP Site 1171). Therefore, these combined observations support the Miocene Palaeoclimate inferences for southwest Australia of Groeneveld et al.

Insights into the variations in the strength and latitudinal extent of the Southern Hemisphere westerlies and their influence on the climate of Australia during the Miocene is provided by Groeneveld et al., In northwest Australia arid conditions were similar to the conditions of the present, and they persisted for most of the Miocene, whereas southwest Australia became continuously wetter during the Late Miocene until ~8 Ma. In southwest Australia the relative arid conditions returned after ~8 Ma and this was synchronous with the drying of Southeast Asia (Zhisheng et al., 2001; Steinke et al., 2010), and the globally increasing of grasses, which are adapted better to arid conditions (Edwards et al., 2010). According to Groeneveld et al. a southwards displacement and weakening of the Hadley cell could have facilitated these changes. It has also been suggested, however, that it is possible these changes could have been related to a major uplift phase of the Himalayas (Clift et al., 2008) or a global decrease in atmospheric decrease in concentrations of CO₂ (Zhang et al., 2013; Gersonde & Censarek, 2006). It is suggested by this study that atmospheric circulation in the Southern Hemisphere is highly sensitive to climate change on longer time scales. It is implied by this that large scale climate change in the future is likely to result in shifts in precipitation patterns in Australia as well as large parts of Southeast Asia.

Groeneveld, J., et al. (2017). "Australian shelf sediments reveal shifts in Miocene Southern Hemisphere westerlies." Science Advances 3(5): e1602567.