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

Abstract

Continuous stable isotope records from marine sediment cores spanning the Pliocene have been used to assess the oceans' response to major perturbations in the climate system as the oceans play an integral role in regulating the global distribution of heat and gases. The Early to mid‐Pliocene has previously been characterized as a time of relative warmth followed by Late Pliocene Southern Hemisphere cooling and bipolar glaciation at ~2.7 Ma. Previous studies have predominantly focused on the Atlantic and Equatorial Pacific Oceans. In this study, we extended the deep water benthic foraminifera stable isotope record from Ocean Drilling Program (ODP) Site 1123 in the southwest Pacific, back to the warm Early Pliocene. This is a high‐latitude site at the gateway where the abyssal waters enter the Pacific Ocean and provides information about the connection between the Southern Ocean and the Pacific. We identify a dichotomy between the deep southwest Pacific and South Atlantic δ13C records spanning the mid‐Pliocene and suggest that this is most likely the result of variations in the relative contributions of Northern versus Southern Hemisphere deep waters to the different basins. At 3.6 Ma, δ13C values start to decrease; this is interpreted to represent alteration in preformed values as a result of increased remineralization of carbon caused by a reduction in deep ocean ventilation in the Southern Ocean. This is likely the consequence of a greater extent and seasonal duration of sea ice in the Southern Ocean from Antarctic Ice Sheet expansion and cooling.

A Ternary Mixing Model Approach Using Benthic Foraminifer δ13C‐δ18O Data to Reconstruct Late Pliocene Deep Atlantic Water Mass Mixing

“…The hypothesized mixing scheme for Sites 704, 1264, and 1267 is illustrated in Figure 6. This interpretation differs substantially from previous work in which only the latitudinal δ 13 C gradients were considered and a high δ 13 C signature in the southeast Atlantic was explained by a strong contribution of NCW (Hodell & Venz, 2006; Patterson et al, 2018). However, the high δ 18 O values in the late Pliocene southeast Atlantic clearly show a different origin for these waters.…”

“…From a long‐term perspective, the best diagnostic for the presence of the relatively cool and well‐ventilated SEADW may be a positive δ 13 C gradient between Site 704 and SCW‐dominated Site 1123, which occurs between 3.6 Ma and 2.7 Ma (Patterson et al, 2018). Before 3.6 Ma, both δ 13 C and δ 18 O were similar at Site 704 and Site 1123, which suggests no distinct SEADW.…”

“…With reduced ice sheets and warmer Southern Ocean temperatures in the Early Pliocene (McKay et al, 2012; Patterson et al, 2018), the waters formed from the ACC may not have been dense enough to sink to the deep ocean. At 2.7 Ma, the δ 13 C gradient between Site 704 and Site 1123 reversed, possibly related to reduced ventilation of the Southern Ocean caused by increased seasonal sea ice (Hillenbrand & Cortese, 2006; McKay et al, 2012; Patterson et al, 2018), in combination with a southward shift of the polar front and a decrease in the easterlies strength (Zhang, Nisancioglu, Chandler, et al, 2013). From this point in time, SEADW can no longer be recognized as a high‐δ 13 C high‐δ 18 O southern‐sourced water mass, although an analog water mass may still have formed by sinking waters in the ACC.…”

“…Yellow stars mark sites for which we present new data. References: Site 959: this study; Site 662: this study and Lisiecki and Raymo (2005); Site 925 and Site 929: Tiedemann and Franz (1997), Billups et al (1998), and Wilkens et al (2017); Site 659: Tiedemann et al (1994) and Wang et al (2010); Site 610: Raymo et al (1992); Site U1308: Hodell and Channell (2016), De Schepper et al (2013), and De Schepper (2015); Site U1313: Bolton et al (2010) and Lang et al (2014); Site 1264 and Site 1267: Bell et al (2014); Site 704: Hodell and Venz (1992); Site 846 and Site 849: Mix et al (1995); Site 1148: Tian et al (2008); Site 1123: Harris (2002) and Patterson et al (2018); Site 758: Chen et al (1995). The map was generated in Ocean Data View (Schlitzer, 2019).…”

On climate and abyssal circulation in the Atlantic Ocean during late Pliocene marine isotope stage M2, ∼3.3 million years ago

“…While its Plio-Pleistocene benthic δ 13 C stratigraphy shows that the composition of SCW bathing the deep Atlantic has changed over the period of our study interval, it also confirms that marked differences persisted in the δ 13 C of NCW and SCW during cold stages over the past 3.6 Ma. The δ 13 C of Site 1123 is thought to have decreased from 3.6 Ma in response to increased carbon remineralisation in SCW caused by Southern Ocean sea-ice expansion and a reduction in deep ocean ventilation (Patterson et al, 2018).…”