Late Holocene Foraminiferal Biofacies from Baffin Bay, Texas: Preliminary Analysis of a 4000-Year Record of Paleosalinity: ABSTRACT

Abstract

ABSTRACT Predominant hypersalinity in Baffin Bay, Texas, leads to the preservation of laminated sediments. These largely unbioturbated sediments permit reconstruction of a high-resolution, late Holocene paleoenvironmental record. Piston cores from the center of Baffin Bay recovered up to 5 m (16.4 ft) of these fine-grained sediments. On the basis of previous dating of dolomite layers and sedimentation rate estimates (ca. 1 mm/yr), these cores represent up to 4,000 yr of deposition. Preliminary micropaleontological analysis of core L593-2 (4.9 m [16 ft] long) is based on 120 samples with an average sample spacing of 3-4 cm. Foraminiferal assemblages are dominated by three major taxonomic groups: (1) Ammonia parkinsoniana; (2) Elphidium spp.; and (3) miliolids (mostly Quinqueloculina spp. and Triloculina spp.). Previous work on modern microfaunas in Texas embayments shows that each of these groups may dominate benthic faunas under different environmental conditions. Miliolids dominate hypersaline environments, whereas the other two groups will tolerate moderately brackish water. Accordingly, core samples dominated by miliolids are interpreted to represent the most hypersaline conditions. Samples dominated by Ammonia parkinsoniana represent the most brackish salinities encountered in the core (but greater than 20 ppt). Dominance of benthic assemblages varies dramatically within the core. Major biofacies trends include (1) no occurrence of either low-salinity, agglutinated or open shelf assemblages; (2) peaks in miliolid abundance that define paleosalinity fluctuations on a scale of 10 to 30 cm in the upper 4.2 m (13.8 ft) of the core (thus representing cyclicity on the scale of hundreds of years); and (3) very rare or no miliolids from 4.2 to 4.9 m, suggesting an absence of hypersalinity, but still oceanographically restricted conditions. The core did not penetrate sediments deposited prior to restriction of Baffin Bay by Padre Island (ca. 5,000 yr B.P. Three dolomite layers within the core (at 2.86 m, 4.34 m, and 4.71 m) occur in intervals with rare or no miliolids, arguing against a hypersaline setting for their formation. Ongoing work on these sediments includes AMS carbon-14 dating to better constrain sediment accumulation rates and quantitative analysis of the benthic foraminiferal data by transfer functions, in order to derive quantitative paleosalinity estimates.