The last deglaciation in Orca Basin, Gulf of Mexico: High-resolution planktonic foraminiferal changes
Abstract Anoxic sediments in Orca Basin, northern Gulf of Mexico (water depth 2400 m) provide late Quaternary planktonic foraminiferal and paleoclimatic records of remarkably high resolution (∼ 250 yrs). Black, organic-rich, strongly laminated, unbioturbated muds of Orca Basin contrast with grayish, organic-poor, bioturbated sediments immediately outside of the basin. Planktonic foraminiferal assemblages contained within totally anoxic sediments exhibit superb preservation and probably have not been changed much, by dissolution or other processes, from the original living assemblage. A quantitative comparison of surface-sediment assemblages (> 100 μ m and > 175 μ m size fractions) from the anoxic area and from the oxygenated regime immediately outside the basin, nevertheless has revealed no significant differences between the assemblages. It is inferred, therefore, that the assemblages well above the lysocline in the northern Gulf have been subjected to little postmortem alteration, even though their preservation does not match those from the Orca Basin. Planktonic foraminiferal assemblages have been counted in two piston cores from Orca Basin that are as old as 29 kyrs B.P., and which contain continuous, high-resolution records of the last deglaciation. The location of these cores is 290 kmS of the modern Mississippi Delta and is known to have been within the influence of meltwater discharge from the Mississippi River system during the last deglaciation of the Laurentide ice sheet, an event which significantly decreased surface-water salinities in the Gulf of Mexico. This meltwater effect is strongly recorded in the Orca Basin sequence. Faunal and stable-isotope changes have been evaluated in relation to the Pleistocene-Holocene transition and are associated with two major processes related to deglaciation: (1) the well-known glacial-to-interglacial surface-water changes as cool Gulf waters were replaced by warmer water masses; and (2) severe open-ocean surface-water salinity decrease resulting from the meltwater discharge into the Gulf between about 16.5 and 12 kyrs ago. Species associations were dynamically changing throughout the late Quaternary in the northern Gulf in response to oceanic and continental influences. Factor analysis of the down-core assemblages reveal three major faunal groupings which account for 95% of the total variance. Factor one is by far dominated by Globigerinoides ruber which shows highest values closely associated with a major negative oxygen isotopic anomaly that resulted from the deglacial meltwater discharge to the Gulf. A severe surface-water salinity decrease created a harsh environment that was not favorable to the successful proliferation of most other planktonic foraminiferal species. This low-salinity fauna interferes with the normal faunal transition observed elsewhere between the latest Pleistocene and Holocene. Factor two is a warm-water (interglacial) assemblage dominated by Globorotalia menardii, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei . Factor three is a cool-water (glacial) assemblage dominated by Globorotalia inflata, Globorotalia crassaformis and Globigerinella aequilateralis . In the sequence, the glacial assemblage is replaced about 16.5 kyrs ago by the low-salinity assemblage which itself persisted until about 12 kyrs ago when the meltwater ceased to flow into the Gulf and was replaced briefly (for 1.5 kyrs) by a return of the glacial assemblage until about 10.5 kyrs ago. This cool interval seems to be correlative with the Younger Dryas cold episode of Europe, and with the cold interval between Termination 1A and 1B in north Atlantic cores. After this, the warm-water (interglacial) assemblage migrated into the Gulf. Early Holocene (10.5-5 kyrs ago) assemblages are quantitatively distinct from late Holocene (5-0 kyrs ago) assemblages. Unlike previously documented associations, G. ruber rather than N. dutertrei , dominated during intervals of low surface-water salinity in the Orca Basin. We believe that surface-water masses were still too cool during the meltwater spike and salinities were too low for the proliferation of N. dutertrei . On the other hand G. ruber , a more euryhaline opportunistic species, survived the severe conditions, but did not prosper. N. dutertrei distinctly increased to moderate frequencies in the second half of the meltwater spike probably as surface waters warmed during deglaciation. In cores more distant from the Mississippi River outfall region, where oxygen isotopic evidence indicates the occurrence of less severe salinity reductions, N. dutertrei exhibits a distinct frequency peak, and G. ruber is less dominant. Species frequency oscillations provide intercore correlations at a resolution of only several hundred years.