Polar Frontal Migration in the Warm Late Pliocene: Diatom Evidence From the Wilkes Land Margin, East Antarctica

Abstract

The late Pliocene is the most recent interval in Earth's history to sustain global temperatures within the range of warming predicted for the 21st century, providing an appealing analog for the changes we might encounter in the coming century. Published global reconstructions and climate models find an average +2° summer sea surface temperature anomaly relative to modern during the 3.3–3.0 Ma interval of the late Pliocene, when atmospheric CO2 concentrations last reached 400 ppm. Here we present a new diatom‐based reconstruction of Pliocene interglacial sea surface conditions from IODP Site U1361, on the East Antarctic continental rise. We find that open ocean conditions in the mid‐Pliocene became increasingly influenced by sea ice from 3.6–3.2 Ma, prior to the onset of Northern Hemisphere glaciation. This cooling trend was interrupted by a temporary southward migration of the Antarctic Polar Front, bathing U1361 in subantarctic waters during a single interglacial—marine isotope stage KM3 (3.17–3.15 Ma)—after which sea ice returned. Building on the identification of this single outlier interglacial, we have revisited earlier reconstructions to explore the response of the Southern Ocean/cryosphere system to peak late Pliocene warmth. By applying a modern chronostratigraphic framework to those low‐resolution reconstructions, we identify the same frontal migration in four other cores in the Pacific sector of the Southern Ocean, documenting a major migration of the polar front during a key interval of warm climate. These new results suggest that KM3 is a crucial interval to test ice sheet stability in the context of anthropogenic warming.

A Southwest Pacific Perspective on Long‐Term Global Trends in Pliocene‐Pleistocene Stable Isotope Records

“…Ultimately, it is difficult to fully quantify the relative increase in NCW into the Southern Ocean sector based on simple comparisons between end member types due to dramatic changes in Southern Ocean surface conditions compared to modern. However, given the growing number of Southern Ocean and Antarctic margin records identifying such differences (e.g., Cook et al, 2013;Naish et al, 2009;Patterson et al, 2014), we suggest that Early Pliocene SCW had higher preformed While the Late Pliocene-Pleistocene transition around 2.7 Ma is a marked time of high-latitude global cooling with increases in continental ice volume and a decrease in global atmospheric CO 2 , a growing number of high southern latitude records infer this cooling trend initiated around 3.6-3.5 Ma (McKay et al, 2012;Riesselman & Dunbar, 2013;Taylor-Silva & Riesselman, 2018). Surface water cooling around the Antarctic margin and in the Southern Ocean during this time have been hypothesized to displace the Sub Antarctic Front (SAF) northward (Taylor-Silva & Riesselman, 2018).…”

“…However, given the growing number of Southern Ocean and Antarctic margin records identifying such differences (e.g., Cook et al, 2013;Naish et al, 2009;Patterson et al, 2014), we suggest that Early Pliocene SCW had higher preformed While the Late Pliocene-Pleistocene transition around 2.7 Ma is a marked time of high-latitude global cooling with increases in continental ice volume and a decrease in global atmospheric CO 2 , a growing number of high southern latitude records infer this cooling trend initiated around 3.6-3.5 Ma (McKay et al, 2012;Riesselman & Dunbar, 2013;Taylor-Silva & Riesselman, 2018). Surface water cooling around the Antarctic margin and in the Southern Ocean during this time have been hypothesized to displace the Sub Antarctic Front (SAF) northward (Taylor-Silva & Riesselman, 2018). While this cooling in surface waters would directly be apparent by changes in SST (i.e., McClymont et al, 2016), it is reasonable to assume based on analogues Last Glacial Maximum (LGM) conditions that there would be associated changes with density stratification between intermediate and deep waters as well as changes in deepsea carbon storage (Bostock et al, 2013;Sigman et al, 2010;Sikes et al, 2016Sikes et al, , 2017.…”

“…While the Late Pliocene-Pleistocene transition around 2.7 Ma is a marked time of high-latitude global cooling with increases in continental ice volume and a decrease in global atmospheric CO 2 , a growing number of high southern latitude records infer this cooling trend initiated around 3.6-3.5 Ma (McKay et al, 2012;Riesselman & Dunbar, 2013;Taylor-Silva & Riesselman, 2018). Surface water cooling around the Antarctic margin and in the Southern Ocean during this time have been hypothesized to displace the Sub Antarctic Front (SAF) northward (Taylor-Silva & Riesselman, 2018).…”

“…Previous work has argued that higher δ 13 C values in the South Atlantic may have resulted from an increase in preformed values of SCW during a time of sustained global warmth and reduced sea ice extent in the Southern Ocean (Hodell & Venz-Curtis, 2006). However, there is now increasing evidence that this was the beginning of a major transition in the AIS dynamics for both the WAIS and the EAIS (McKay et al, 2012;Naish et al, 2009;Patterson et al, 2014) as well as surface ocean conditions in the Southern Ocean (Taylor-Silva & Riesselman, 2018). Direct evidence from marginal marine records in the Pacific Ocean sector infer expansion of the marine-based ice sheet in the Ross Sea (Naish et al, 2009) and a threshold response to marine-based margins of the EAIS to orbital scale perturbations in response to feedback relating to more extensive summer sea ice (Patterson et al, 2014).…”

The Variable and Changing Southern Ocean Silicate Front: Insights From the CESM Large Ensemble

“…The modern‐day relationship between diatom productivity and export in the silicate‐replete Antarctic waters south of the PF and the opal‐dominated sediments below (i.e., the Opal Belt; e.g., DeMaster, ) has been used to infer changes in diatom productivity and the air‐sea carbon balance on glacial‐interglacial time scales (e.g., Abelmann et al, ; Anderson et al, ; Crosta et al, ; Kemp et al, ; Kohfeld et al, ; Nair et al, ; Taylor‐Silva & Riesselman, ). Whether the PF was displaced significantly north or south from its modern‐day position on these time scales is still a topic of debate (see Kemp et al, ).…”