The mid-Pliocene (4.3–2.6 Ma) benthic stable isotope record of the Southern Ocean: ODP Sites 1092 and 704, Meteor Rise

Abstract

We present mid-Pliocene (4.3–2.6 Ma) benthic stable oxygen and carbon isotope data from Ocean Drilling Program Site 1092 (ODP Leg 177) drilled in the sub-Antarctic sector of the Southern Ocean. The results are compared with the stable isotope results from nearby Site 704 (ODP Leg 114). Oxygen isotope data show that minimum values are about 0.5‰ less than those of the Holocene, which is consistent with the results from Site 704, indicating only minor deglaciation of Antarctica during the studied interval. Oxygen isotope data from both Site 1092 and Site 704 are slightly higher relative to Pacific values during several intervals which could be related to the contribution of warm, saline North Atlantic Deep Water (NADW). Comparisons of benthic carbon isotope gradients between sites located in the North Atlantic, sub-Antarctic sector of the Southern Ocean, and Pacific indicate that at times, the gradient between the Southern Ocean and the Pacific evolved differently than the Atlantic–Pacific gradient. This suggests that variations in NADW strength alone might not be responsible for the observed carbon isotope values in the Southern Ocean.

Introduction

The early Pliocene is generally considered to be the most recent period with a global climate warmer than today (e.g. Crowley, 1991 and references therein). Global temperature may have been 3.5°C warmer than at present prior to the onset of Northern Hemisphere Glaciation (Raymo et al., 1996). For the Northern Hemisphere there is evidence of formation of sea ice and snow cover in the Arctic and sub-Arctic regions as early as in the Late Miocene (Jansen et al., 1990, Jansen and Sjøholm, 1991) but marked expansion of large ice sheets is not evident until 3.3 Ma except for Greenland (Jansen et al., 2000)

Although a general consensus exists that the high-latitude Southern Hemisphere was warmer during parts of the Pliocene, there has been much debate concerning the dynamics of the East Antarctic ice sheet and its response to early Pliocene warmth. An essentially stable East and West Antarctic Ice Sheet (EAIS) during most of the Pliocene has been advocated by e.g. Clapperton and Sugden (1990), Kennett and Barker (1990), Hodell and Warnke (1991), Warnke et al. (1996) and Murphy et al. (2002), whereas others have supported the idea of a highly dynamic ice sheet during the Early and early Late Pliocene, e.g. Webb and Harwood (1991) and Hambrey and Barrett (1993). One of the more important lines of evidence for EAIS instability during the Pliocene came from marine diatoms found in the Sirius Group tills (Webb and Harwood, 1991, and references therein). According to the deglaciation theory, the present EAIS must post-date the youngest marine incursion, which could be identified by marine diatom being found in the Sirius Group tills. However, other works have proposed a non-marine origin of the Sirius till diatoms. Denton et al. (1991) proposed an eolian origin of the diatoms found in the Sirius Group tills. Burckle and Potter (1996) found that Pliocene–Pleistocene diatoms are present in Paleozoic and Mesozoic sedimentary rocks as well as igneous rocks exposed in East Antarctica, hence, these diatoms can not be used as evidence for a melt-down and collapse of the EAIS in the mid-Pliocene. Furthermore, studies based on isotopic and sedimentological analyses of deep-sea sediments in the Southern Ocean indicate that despite Pliocene warmth, the EAIS remained essentially intact during the Pliocene (Hodell and Venz, 1992, Burckle et al., 1996, Warnke et al., 1996).

Although the origin of Northern Hemisphere Glaciation remains uncertain, it has been shown that the Earth’s climate was closely linked to changes in thermohaline circulation during the Pliocene and Pleistocene (e.g. Oppo and Fairbanks, 1987, Raymo et al., 1990, Raymo et al., 1992, deMenocal et al., 1992, Oppo et al., 1995). Thermohaline circulation plays an important role in meridional heat transport as well as in the modulation of CO₂ exchange between the deep ocean carbon reservoir and the atmosphere. Today the thermohaline circulation is largely driven by sinking of cold, dense water in the North Atlantic and in the Southern Ocean. In the North Atlantic, warm thermocline water with high salinity is cooled as it moves northward into the Norwegian–Greenland seas and the Labrador Sea. At these locations, the now denser water masses sink, forming southward flowing North Atlantic Deep Water (NADW) (Broecker and Denton, 1989). The high initial carbon isotope signature of NADW can be traced as it travels southward (Kroopnick, 1985). Oppo and Fairbanks (1987) showed that the relative flux of NADW out of the Atlantic could be monitored in the Southern Ocean where the high δ¹³C NADW mixes with recirculated Pacific water with low δ¹³C ratio. δ¹³C measurements in deep ocean cores located in the Southern Ocean may act as monitors of the relative input of NADW (Hodell, 1993, Oppo and Fairbanks, 1987, Oppo and Fairbanks, 1990, Oppo et al., 1990, Raymo et al., 1992).

The recovery of continuous sequences in the Southern Ocean is difficult due to the presence of hiatuses and sometimes severe carbonate dissolution. In addition, the often harsh weather conditions in this region affect the drilling operations. To date only one site in the Atlantic sector of the Southern Ocean (Ocean Drilling Program (ODP) Site 704) has had sufficient continuity to yield a high-resolution isotope stratigraphy (Hodell et al., 1991, Hodell and Venz, 1992, Hodell, 1993). However, Site 704 was drilled in a small sedimentary basin on the southern part of Meteor Rise, surrounded by topographic highs which raise concerns about possible downslope transport at the location of Site 704. Indeed, seismic profiles in the vicinity of Site 704 show evidence of a turbidite sequence about 25 mbsf (Gersonde et al., 1999). The results presented by Mix et al. (1995) also raise some concern around the Site 704 data. Mix et al. (1995) found that in the Early Pliocene, below a hiatus at Site 704 between ∼2.44–2.77 Ma, the δ¹⁸O values from Site 704 are higher than those of Site 849 by an average of 0.6%. Since Site 849 is situated at a deeper depth than Site 704 (Table 1) the oxygen isotope data suggest that the deep Pacific was warmer and fresher than the shallower sub-Antarctic Southern Ocean, which is an unlikely oceanographic scenario.

Site 1092 was drilled with the aim of recovering sediments from an area less likely than Site 704 to be influenced by downslope transport. A broad, shallower area was selected on the Meteor Rise based on detailed seismic surveys and the results from Site PS2083-3, a piston core recovered just northwest of Site 1092 (Bathmann et al., 1992). The primary goal of this paper is to present a benthic stable isotope record for the Atlantic sector of the Southern Ocean for the Gauss and late Gilbert chrons. We have analyzed δ¹⁸O and δ¹³C at an average sample spacing of about 6.5 kyr in sediments recovered from ODP Site 1092 (46°24.7′S, 7°4.8′E, 1974 m water depth). We compare these data with isotopic measurements from ODP Site 704 (Hodell and Venz, 1992), and look at the δ¹⁸O and δ¹³C gradients between the North Atlantic, Southern Ocean and Pacific during the Gauss and late Gilbert chrons. Table 1 lists the location of sites discussed in the text.