ALBERT

Description: During the Mid-Pleistocene Transition (MPT), the dominant glacial-interglacial cyclicity as inferred from the marine d18O records of benthic foraminifera (d18Obenthic) changed from 41 kyr to 100 kyr years in the absence of a comparable change in orbital forcing. Currently, only two Mg/Ca-derived, high-resolution bottom water temperature (BWT) records exist that can be used with d18Obenthic records to separate temperature and ice volume signals over the Pleistocene. However, these two BWT records suggest a different pattern of climate change occurred over the MPT-a record from North Atlantic DSDP Site 607 suggests BWT decreased with no long-term trend in ice volume over the MPT, while South Pacific ODP Site 1123 suggests that BWT has been relatively stable over the last 1.5 Myr but that there was an abrupt increase in ice volume at ~900 kyr. In this paper we attempt to reconcile these two views of climate change across the MPT. Specifically, we investigated the suggestion that the secular BWT trend obtained from Mg/Ca measurements on Cibicidoides wuellerstorfi and Oridorsalis umbonatus species from N. Atlantic Site 607 is biased by the possible influence of D[CO3]2- on Mg/Ca values in these species by generating a low-resolution BWT record using Uvigerina spp., a genus whose Mg/Ca values are not thought to be influenced by D[CO3]2-. We find a long-term BWT cooling of ~2-3°C occurred from 1500 to ~500 kyr in the N. Atlantic, consistent with the previously generated C. wuellerstorfi and O. umbonatus BWT record. We also find that changes in ocean circulation likely influenced d18Obenthic, BWT, and d18Oseawater records across the MPT. N. Atlantic BWT cooling starting at ~1.2 Ma, presumably driven by high-latitude cooling, may have been a necessary precursor to a threshold response in climate-ice sheet behavior at ~900 ka. At that point, a modest increase in ice volume and thermohaline reorganization may have caused enhanced sensitivity to the 100 kyr orbital cycle.

Description: As global climate warms and sea level rises, coastal areas will be subject to more frequent extreme flooding and hurricanes. Geologic evidence for extreme coastal storms during past warm periods has the potential to provide fundamental insights into their future intensity. Recent studies argue that during the Last Interglacial (MIS 5e, ~128-116 ka) tropical and extratropical North Atlantic cyclones may have been more intense than at present, and may have produced waves larger than those observed historically. Such strong swells are inferred to have created a number of geologic features that can be observed today along the coastlines of Bermuda and the Bahamas. In this paper, we investigate the most iconic among these features: massive boulders atop a cliff in North Eleuthera, Bahamas. We combine geologic field surveys, wave models, and boulder transport equations to test the hypothesis that such boulders must have been emplaced by storms of greater-than-historical intensity. By contrast, our results suggest that with the higher relative sea level (RSL) estimated for the Bahamas during MIS 5e, boulders of this size could have been transported by waves generated by storms of historical intensity. Thus, while the megaboulders of Eleuthera cannot be used as geologic proof for past "superstorms," they do show that with rising sea levels, cliffs and coastal barriers will be subject to significantly greater erosional energy, even without changes in storm intensity.

Description: The Last Interglacial (MIS 5e, 128-116 ka) is among the most studied past periods in Earth's history. The climate at that time was warmer than today, primarily due to different orbital conditions, with smaller ice sheets and higher sea-level. Field evidence for MIS 5e sea-level was reported from thousands of sites, but often paleo shorelines were measured with low-accuracy techniques and, in some cases, there are contrasting interpretations about paleo sea-level reconstructions. For this reason, large uncertainties still surround both the maximum sea-level attained as well as the pattern of sea-level change throughout MIS 5e. Such uncertainties are exacerbated by the lack of a uniform approach to measuring and interpreting the geological evidence of paleo sea-levels. In this review, we discuss the characteristics of MIS 5e field observations, and we set the basis for a standardized approach to MIS 5e paleo sea-level reconstructions, that is already successfully applied in Holocene sea-level research. Application of the standard definitions and methodologies described in this paper will enhance our ability to compare data from different research groups and different areas, in order to gain deeper insights into MIS 5e sea-level changes. Improving estimates of Last Interglacial sea-level is, in turn, a key to understanding the behavior of ice sheets in a warmer world.

Description: For nearly a century, the Atlantic Coastal Plain (ACP) of the United States has been the focus of studies investigating Pliocene and Pleistocene shorelines, however, the mapping of paleoshorelines was primarily done by using elevation contours on topographic maps. Here we review published geologic maps and compare them to paleoshoreline locations obtained through geomorphometric classification and satellite data. We furthermore present the results of an extensive field campaign that measured the mid-Pliocene (~ 3.3-2.9 Ma) shorelines of the Atlantic Coastal Plain using high-accuracy GPS and digital elevation models. We compare our new dataset to positions and elevations extracted from published maps and find that the extracted site information from earlier studies is prone to significant error, both in the location and, more severely, in the elevation of the paleoshoreline. We also investigate, using geophysical modeling, the origin of post-depositional displacement of the shoreline from Georgia to Virginia. In particular, we correct the elevation of our shoreline for glacial isostatic adjustment (GIA) and then compare the corrected elevation to predictions of mantle flow-induced dynamic topography (DT). While a subset of these models does reconcile the general trends in the observed elevation of the mid-Pliocene shoreline, local discrepancies persist. These discrepancies suggests that either (i) the DT and GIA models presented here do not capture the full range of uncertainty in the input parameters; and/or (ii) other influences, such as sediment loading and unloading or local fault-driven tectonics, may have contributed to post-depositional deformation of the mid-Pliocene shoreline that are not captured in the above models. In this context, our field measurements represent an important observational dataset with which to compare future generations of geodynamic models. Improvements in models for DT, GIA and other relevant processes, together with an expanded, geographically distributed set of shoreline records, will ultimately be the key to obtaining more accurate estimates of eustatic sea level not only in the mid-Pliocene but also earlier in the Cenozoic.