Biogenic carbonate fluxes and preservation in the northwestern Labrador Sea since the Last Glacial Maximum

The important changes that took place in the glacial cycle at the termination, from the Last Glacial Maximum to the present interglacial, deserve an examination of ocean sedimentary records that document past productivity, carbon fluxes, and carbonate preservation. In this study, we analyzed coccoliths, alkenones, and foraminifers in core HU2008-029-004 PC (61.46 degrees N and 58.04 degrees W, water depth = 2,674 m) from the northwestern Labrador Sea to document linkages between hydrographic conditions, biogenic carbonate fluxes to the seafloor, and their preservation/dissolution during the last 25,000 years. Large changes in coccolith and foraminifer concentrations are recorded, with sediments from the last glacial interval containing significantly less carbonate micmfossils (9.5 +/- 3.9 x 10(5) coccoliths g(-1) and 2,860 +/- 580 planktonic foraminifers g(-1)) than sediments from the deglacial and postglacial intervals (up to 3.1 x 10(8) coccoliths g(-1) and 2.9 x 10(4) foraminifers g(-1)). Three foraminifer-based calcite dissolution indices were used to evaluate biogenic carbonate preservation: the planktonic foraminifer fragmentation index, the ratio of benthic-to-planktonic foraminifers (B/P), and the ratio of organic linings to benthic foraminifers (OL/B). Fragmentation remained low throughout the postglacial (mean of 4%) but reached up to 8% in the deglacial and peaked at 16% in samples from the Bulling-Allerud of the late glacial interval. Samples from the Bulling-Allerud and the deglacial interval also display a slightly elevated B/P index (>0.15), which suggests that some dissolution may have occurred. In contrast, with the exception of the Bulling-Allerud and the deglacial interval, near zero OL/B values characterize most of the sequence, suggesting good biogenic carbonate preservation, which implies that the low biogenic carbonate and coccolith content in sediments of the glacial stage mirror low productivity of calcifying organisms. The elevated fragmentation of foraminifers during the Bulling-Allerud and the deglacial interval, a time of elevated productivity and low percentages of ice-rafted debris, may indicate the development of calcite undersaturated porewaters and consequent dissolution resulting from oxic remineralization of sedimentary organic matter. We also identify a significant decoupling of coccolith and alkenone concentrations throughout the core. Colder-than-expected U-37(K)-SST estimates from the alkenones of the glacial interval rule out possible allochthonous inputs from lower-latitude locations. Instead, our records imply that at least during the glacial interval, alkenones were produced by non-calcifying haptophytes that may not follow the canonical U-37(K)-based temperature calibrations.

Automated summary

This study analyzed ocean sedimentary records from the northwestern Labrador Sea to examine past productivity, carbon fluxes, and carbonate preservation over the last 25,000 years. The findings show significant changes in biogenic carbonate fluxes and preservation during the last glacial interval, indicating lower productivity of calcifying organisms.