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doi:10.2204/iodp.proc.303306.214.2010

Conclusions

The overall objective of Expedition 303/306 is to generate high-resolution records of late Miocene-Quaternary North Atlantic environmental change that can be placed in a high-resolution stratigraphic framework based on oxygen isotope and RPI data.

Apart from Site U1312, where coring disturbance attributed to severe weather conditions contributed to poor preservation of the record, paleomagnetic records from Expedition 303/306 sites are of high fidelity. In general, polarity chrons are well defined in shipboard data. Two postcruise studies of the details of polarity transitions attest to the quality of the magnetic records (Ohno et al., 2008; Mazaud et al., 2009). Isotope records are still in development at most Expedition 303/306 sites, although a coupled isotope/RPI record has been published for Site U1308 for the last 1.5 m.y. (Hodell et al., 2008; Channell et al., 2008) and this stratigraphy has been augmented by detailed nannofossil stratigraphy at this site (Sato et al.; Chiyonobu et al.). The benthic isotope and RPI records at Site U1308 over the last 1.5 m.y. have high resolution and fidelity and provide the reference records for coupled RPI/isotope stacks generated by tandem correlation of isotope and RPI data from 13 globally distributed records (Channell et al., 2009b). The tandem correlations use the Match protocol (Lisiecki and Lisiecki, 2002) that allows repeatable correlations using penalty functions that can be set to inhibit abrupt sedimentation rate changes within the records. This is a first step toward generating a template for stratigraphic correlation at a resolution not obtainable from oxygen isotopes alone. This development is important for the study of millennial-scale environmental change because the lack of tools for stratigraphic correlation, at an appropriate resolution, is presently a severe impediment to the study of phenomena associated with short-term “abrupt” climate change.

The detrital layer stratigraphy at Site U1308 has been resolved using density, magnetic susceptibility and XRF scanning, specifically the Ca/Sr ratio, which is a valuable proxy for Hudson Strait–derived (Heinrich-type) detrital layers because of the low Sr content of inorganic (detrital) versus biogenic carbonate (Hodell et al., 2008). In addition, the distinctively lower δ¹⁸O values for detrital carbonate, relative to foraminifer calcite, means that bulk carbonate δ¹⁸O provides an additional means of recognizing detrital events (Hodell and Curtis, 2008). Low δ¹⁸O values from planktonic foraminifer tests contaminated by adhered detrital carbonate may mimic meltwater events. Hudson Strait–derived Heinrich events, characterized by detrital carbonate, appear in MIS 8, 10, 12, and 16 but not, apparently, in MIS 6 or 14 (Hodell et al., 2008).

At Site U1313, Heinrich-type layers characterized by lithics with detrital carbonate occurred in glacial MIS 10, 12, and 16 as well as at Terminations V and VII (Stein et al., 2009; Voelker et al., 2010). High-resolution isotopic, alkenone, and XRD studies at Site U1313 in the 320–640 ka (MIS 9–16) interval have allowed detailed comparison of SST variability, surface water productivity, and IRD deposition in the central mid-latitude North Atlantic (Stein et al., 2009; Voelker et al., 2010). Peaks in dolomite and feldspars and associated maxima in land-plant derived n-alkanes are interpreted as indicators of Heinrich-like events related to instability of the LIS, especially during early and late stages of glacial intervals (Stein et al., 2009).

Hudson Strait–derived Heinrich-type detrital layers first appear in the sedimentary records at Sites U1308 and U1313 in MIS 16, approximately coincident with the end of the mid-Pleistocene climate transition between the so-called 41 and 100 k.y. worlds. Although iceberg survivability may play a role in the apparent onset of this IRD record, it is more likely that ice volume and glacial stage duration surpassed critical thresholds at that time that activated LIS instability (Hodell et al., 2008). Benthic δ¹³C variations at Site U1308 support a link between iceberg discharge and weakening of thermohaline circulation.

Planktonic and benthic microfossils at Site U1314 have yielded valuable proxies for surface (radiolarian and calcareous nannofossils) and deep (ostracodes) water conditions in the North Atlantic. Radiolarian assemblages indicate warm SSTs during MIS 6 relative to other Brunhes glacial stages (K.R. Bjørklund et al., unpubl. data), whereas ostracode assemblages and species diversity can be associated with changes in deep ocean circulation and climate variability (Alvarez Zarikian et al., 2009).

The results from Sites U1308, U1313, and U1314 are a foretaste of what will be forthcoming from the group of sites occupied during Expedition 303/306. The preliminary data indicate that all sites have the attributes to provide records of North Atlantic environmental change that can be placed in a chronological framework based on oxygen isotope data and RPI.

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