Refining interpretations of a deglacial record in Gulf of Mexico marine sediment cores using foraminifera C-14 Dates and multi-proxy sediment analyses
The upper fourteen sections of three cores recovered from the International Marine Past Global Changes Study (IMAGES) VIII/Paleoceanography of the Atlantic and Geochemistry (PAGE) 127 expedition of 2002, are stratigraphic records of the climate and provenance variations that influenced sediment deposition in the Gulf of Mexico during the Pleistocene and Holocene. Though originally cored by the USGS for gas hydrate investigations, cores MD02-2535, MD02-2555, and MD02-2560 have been repurposed for undergraduate course-based paleoclimate research at JMU. Shipboard interpretations of the color reflectance data for these cores suggested that two dark stratigraphic intervals present in all of the cores were regional correlative time horizons and marked the Last Glacial Maximum (LGM), ~18-24 ka, and the Younger Dryas (YD) cooling event, ~12.85-11.65 ka (Bout-Roumazeilles and Trentesaux, 2007). Results from previous JMU research for the sediment composition, grain size, C-14 dates and calculated sedimentation rates, led to a re-interpretation with the main dark interval as meltwater pulse 1A (MWP-1A), ~15.4-12.9 ka, instead of the LGM and the smaller dark interval above it as meltwater pulse 1B (MWP-1B), ~11.45-11.1 ka, instead of the YD (Melander et al., 2017; Richardson et al., 2018; Nebel et al., 2019).
This research tests the USGS hypothesis that the dark stratigraphic intervals in these cores are regional time correlative intervals and represent the LGM and YD, with an alternative hypothesis that these units instead correspond to meltwater pulses 1A and 1B from the deglaciation of the Laurentide Ice Sheet. Building on previous JMU work, this research has further improved the age model for all three cores using additional C-14 dates and gathered sedimentological data for core MD02-2555 to refine our understanding of the impact of deglaciation within the Gulf of Mexico. Conclusions from the new age model are that the original USGS interpretations were (1) correct in identifying the shallower interval of low color reflectance as a regional correlative time horizon across all three cores; (2) correct in identifying the deeper interval of low color reflectance as a regional correlative time horizon across cores MD02-2535 and MD02-2555, but slightly too deep for core MD02-2560; but (3) an overestimation of absolute age of sediments at these intervals across all three cores.
Rather, the refined age model supports earlier JMU research that the sediment in the dark intervals across cores MD02-2535, MD02-2555, and MD02-2560 is much younger than the original USGS interpretation (Bout-Roumazeilles and Trentesaux, 2007). The deeper interval of low color reflectance identified in the color reflectance data by the USGS scientists is closer in time to the Bølling-Allerød event instead of the LGM. The shallower interval of low color reflectance is slightly younger than the YD at ~11-10 ka based on the newest age model. Sediment from the dark intervals in core MD02-2555 are enriched in smectite, dominantly finer grained, have low observed biogenic carbonate, have peaks in terrigenous elemental indicators, and are of the same age as MWP-1A and MWP-1B. Based on these observations, it has been concluded that the low color reflectance of these two intervals is likely the result of meltwater influence on sedimentation during deglaciation.