FORAMINIFERAL EVIDENCE OF A LATE HOLOCENE WESTWARD SHIFT OF THE ALEUTIAN LOW PRESSURE SYSTEM

Abstract

The late Holocene distribution of agglutinated foraminifera and freshwater thecamoebians was documented within a 12.26-m piston core (VEC02A04) deposited between 1090 to ~4560 cal. yr BP, collected from the glacier-carved Frederick Sound, Seymour-Belize Inlet Complex (SBIC), British Columbia. Core sediments contained unevenly distributed, massive and laminated intervals interrupted by occasional slumps and turbidites. The microfauna was characterized by low diversity and low abundances of agglutinated foraminifera (Shannon Diversity Index ≤1.36) with rare allochthonous freshwater thecamoebians. The predominance of low-oxygen-tolerant agglutinated foraminifera, and absence of calcareous species indicate that dysoxia was the main environmental stressor. Abundant organic matter in the core sediments provides additional evidence of low-oxygen conditions that excluded most organisms that might have utilized this rich food resource. A moderate up-core increase in the abundance of glaciomarine indicator species (e.g., Spiroplectammina biformis , Recurvoides turbinatus , Portatrochammina bipolaris , and Cribrostomoides jeffreysii ) after 3125 cal. yr BP (above 7.19 m) indicates a subtle shift to progressively cooler bottom-water conditions. The presence of varying proportions of thecamoebians through this upper core interval was likely related to increased spring snowmelt or early summer precipitation, resulting in increased terrestrial erosion (e.g., reworking of marsh sediments into the fjord). Development of relatively cooler/drier climate conditions in the SBIC during the late Holocene corresponds to the neoglacial advances in the NE Pacific, which is directly linked to a weakened and/or westward shift in the center of action of the Aleutian Low Pressure System.