Preservation of aragonite in Late Pleistocene sediments in the deep basin of the Western Gulf of Mexico

Abstract

The Late Pleistocene sediments in the deep basin of the Gulf of Mexico are marked by several episodes of the preservation of aragonitic pteropods. Pteropods are not preserved on the sea floor of the abyssal Gulf today nor were they preserved during previous interglacial periods. In three cores taken from the abyssal Gulf pteropods were found in discrete intervals between approximately 11,000-17,000 years ago. These intervals contain well-preserved planktonic Foraminifera and total organic carbon values 50-75% less than other sediments deposited during the Late Pleistocene (zone Y); the organic carbon is 2-4('o)/oo enriched in ('13)C compared to other sediments deposited during the upper Y zone indicating that the decrease in organic carbon may be due to a cessation in the flux of terrestrial carbon. These data suggest that pteropod preservation could have occurred because of a decrease in the oxidation of organic matter at the sediment-seawater interface thereby rendering bottom waters more conducive to the preservation of carbonate. Factor analysis of benthic Foraminifera from a core in the northwestern Gulf at 1,600 m depth identified 7 assemblages which accounted for 84% of the original data. Of these seven the first three correlate with dissolution intensity. The benthic Foraminifera in the three cores taken from abyssal depths are all species which are commonly found within North Atlantic Deep Water. The glacial assemblage is essentially the same as benthic Foraminifera found at abyssal depths today. The (delta)('13)C record based on U. peregrina was obtained from this same core for the last 130,000 years. This record suggests that during isotope substages 5e to 5c, stage 3, and in the middle of stage 2 carbon isotope values in the Gulf of Mexico were more negative than those found in the North Atlantic. The negative carbon isotope excursions in the Gulf record were between 0.3-0.4('o)/oo, which is the (delta)('13)C difference between NADW and AAIW today. The excursions occurred during periods when it has been suggested by previous workers that NADW production decreased. The resulting decrease in the flux of NADW into the Gulf of Mexico may have permitted AAIW to extend its present depth range. However, there were periods when the bottom waters of the Gulf of Mexico apparently were not affected by inferred changes in NADW production (during isotope stage 4 and substage 5e). This suggestes that those changes in NADW production affected only depths greater than the sill depth in the Gulf of Mexico.