Elsevier

Global and Planetary Change

Volume 69, Issue 3, November 2009, Pages 124-132
Global and Planetary Change

Chronostratigraphic and paleoenvironmental constraints derived from the 87Sr/86Sr and δ18O signal of Miocene bivalves, Southern McMurdo Sound, Antarctica

https://doi.org/10.1016/j.gloplacha.2009.05.001Get rights and content

Abstract

87Sr/86Sr measurements were carried out on samples of various macrofossil taxa recovered from the SMS AND-2A core with the main goal of improving age control of the core, and provide insight on marine climate at the time of carbonate precipitation. Shell material was carefully screened using cathodoluminescence (CL) to discern optimum areas for microdrilling. Powders thus obtained were in addition analyzed for Ca, Mg, Sr, Mn, and Fe contents to further evaluate the possibility of diagenetic alteration of the skeletal carbonate before measuring their Sr isotope compositions. Bivalves turned out to be the best candidates for isotopic determinations. Unaltered calcitic bivalves produced reliable 87Sr/86Sr age ranges. Aragonitic material, on the other hand, produced less radiogenic 87Sr/86Sr than anticipated, an unexpected result. Preliminary oxygen isotope determinations in calcite samples confirm contrasting marine climate conditions between the late Early Miocene (16.5–16.0 Ma), and the early Late Miocene (~ 11 Ma).

Introduction

Interactions among the various components of the climate system are diverse and complex. While definite climate variability has been captured within historical boundaries, this timeframe is insufficient to recognize, evaluate, or predict changes. The modern cryosphere offers the opportunity to study direct climate records at a longer time scale. For example, examining the overall 105 ka record of the Vostok and Dome C ice-cores from East Antarctica (Barnola et al., 1987, Petit et al., 1999, Augustin et al., 2004), the potential effects of the remarkable increase in anthropogenic CO2 which occurred during the last century became apparent. Even though some research suggests decoupling between global temperature trends and atmospheric greenhouse gas concentrations at longer time scales (Veizer et al., 2000), the Vostok and similar ice-core records leave no question about the covariant behavior of these two factors in the short run, and the immediacy of future climate warming (see Crowley and Berner, 2001, IPCC, 2001, IPCC, 2007).

The extent of warming and its effect on other climate components, however, remains unclear. A better understanding of future climate comes only from the deeper geologic past, where a wider range of possible behaviors can place present-day perturbations into context. A sense of urgency stems from the global temperature increase predicted for this century (Cubasch et al., 2001, Meehl et al., 2007), the much faster reduction in the past few years of the Arctic sea ice (Parkinson and Cavalieri, 2008), and the recent evidence contradicting the assumption that Antarctica was somewhat insensitive to the recent global temperature change (Gillett et al., 2008).

Computer modeling has become a powerful predictive tool for climate. A complete understanding of forcing, feedback and thresholds is necessary to produce accurate predictions. Two types of models are typically built in climate studies: parameterized models, such as the widely used global circulation models (GCM), apply initial and boundary conditions to produce a temperature outline that represents climate conditions (e.g., DeConto and Pollard, 2003a, DeConto and Pollard, 2003b, Pekar and DeConto, 2006, Pollard and DeConto, 2009); statistical models, on the other hand, extract and constrain the periodic or non-periodic nature of climate response from temperature records in search of pairing evidence with forcing (e.g., Alley et al., 2001). Success of either one depends on the accuracy of the data feeding the models or against which results are compared. Therefore, continued research to improve and expand proxies' accuracy and reliability is critical to climate understanding and eventual prediction.

Climate proxies in the sedimentary record are abundant and have helped piece together a geologic history of climate change (e.g., Zachos et al., 2001, Lear et al., 2002, Billups and Schrag, 2003). However, the many unanswered questions that remain require focusing research effort on critical areas and time intervals. Antarctica's ice sheets have played a preponderant role on climate through changes in albedo, sea level, and oceanic and atmospheric circulation, among others, and are often called upon to explain remote evidence of climate change (e.g., Harwood et al., 2005). Yet, their behavior under unprecedented rapid global warming is unknown, and sedimentary records from this area during critical intervals are fundamental. Antarctica's climate evidence from the Neogene is particularly important. At this time, the overall Cenozoic cooling trend culminates with the apparent consolidation of both East and West Antarctica ice sheets in the southern hemisphere and the development of permanent ice caps in the northern hemisphere. Understanding the responses and feedbacks established during these events is critical to calibrate climate models and to strengthen their predictive power.

The nature and timing of the change to frigid polar condition in Antarctica during the Miocene is a persistent problem. Global deep-sea δ18O records indicate a transition from a climatic optimum at 17 to 15 Ma, to perennial ice at about 10 Ma (Zachos et al., 2001), while evidence from other sources suggests that this transition occurred later during the Pliocene (Webb and Harwood, 1991, Harwood and Webb, 1998, Murphy et al., 2002). This latter view is supported by evidence from the northern hemisphere that indicates a mid-Pliocene climatic optimum at about 3 Ma, requiring higher sea-levels by up to 50 m shortly before the onset of the northern hemisphere glaciations (Haq et al., 1987, Crowley, 1991, Cronin and Dowsett, 1993, Dowsett et al., 1999). This scenario, which requires substantial melting of Antarctica's ice sheets at the time (e.g., Webb et al., 1984, Raymo et al., 2006), is not without controversy (Kennett and Hodell, 1995, Miller and Mabin, 1998, McKay et al., 2008). Understanding these transitions and assessing climate stability during the past 20 Ma from the Antarctica perspective is relevant to estimate future climate response.

In this paper, 87Sr/86Sr and δ18O from selected Miocene to Pleistocene fossil material recovered by ANDRILL from Antarctica's continental shelf are used to pursue two main goals: first, to contribute to the critical chronostratigraphic control of the section through Sr isotope ages of unaltered samples; and second, to provide insight on local climate based on the environmental limitations imposed by preliminary oxygen isotope data and Sr isotopic composition and concentration in the samples analyzed. Establishing a valid chronostratigraphic framework is critical to enhance interpretations based on this newly retrieved sequence, and though not providing a complete stratigraphy, Sr isotope control can be critical to corroborate results from other dating methods. Similarly, incipient results based on limited oxygen and strontium isotopic composition serve to confirm environmental variability during the Miocene and to provide some rough estimates of departures from global averages.

Section snippets

Regional setting – Core AND-2A

The Antarctic Geological Drilling (ANDRILL) program is an ongoing multinational collaborative effort to drill Antarctica's margin (Florindo et al., in press). ANDRILL's goal is to recover complete enough stratigraphic records to help understand the history of glaciations and environmental change in the Victoria Land Basin region (Fig. 1). This pursuit started with the completion in 2006 of core MIS AND-1B drilled over McMurdo Ice Shelf, and followed in 2007 by the recovery of core AND-2A.

Materials and methods

A total of 19 samples were analyzed for this study. Table 1 provides depth and a brief field description of the samples and of the 31 subsamples that produced 87Sr/86Sr results. Sr isotope values are also shown. These 19 samples were carefully selected from the AND-2A units with the best fossil content and preservation potential. All fragments, bivalves from LSU 7 in particular, showed no signs of significant transport.

Samples were screened using cathodoluminescence (CL) to evaluate the

Results

Cathodoluminescence and chemical analysis showed that alteration, though notable in several intervals, is not a pervasive problem throughout the core. Table 1 shows the metal to Ca ratios for all subsamples and a letter to qualify CL as bright (B), dull (D) or non-luminescent (N). Uncharacteristic luminescence is described with X. Bold text indicates values within range of comparable modern unaltered material. Concentrations are reported using millimoles of metal to moles of Ca. This makes

Discussion

Among pectinids, only samples 4-3, 6 and 13 were judged to be substantially altered. Sample 6 was not positively identified and could belong to a different taxon. Sample 4-3 was taken from the outer layer of the shell. Elemental chemistry of subsamples 8-1 and 8-2 indicates marginal alteration and their age estimate is probably not reliable despite their relatively high Mg and Sr contents. The possible deviation from pristine values is not quantifiable. Sample 8, a very small, highly

Conclusions

Reliable Sr ages are obtained from well preserved fossil samples recovered from AND-2A, in particular from calcitic bivalves. Pectinid samples help to further establish age control at 144.05, 366.8 and 430.6 mbsf in accord with the core's age model (Acton et al., in press). CL serves to preliminarily assess diagenetic alteration of carbonate fossils, but may not suffice to understand samples with more complex chemical alteration. With the exception of echinoid fragments, samples with high

Acknowledgements

The ANDRILL project is a multinational collaboration between the Antarctic programmes of Germany, Italy, New Zealand and the United States. Antarctica New Zealand is the project operator and developed the drilling system in collaboration with A. Pyne. Antarctica New Zealand supported the drilling team at Scott Base; Raytheon Polar Services Corporation supported the science team at McMurdo Station and the Crary Science and Engineering Laboratory. The ANDRILL Science Management Office at the

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