Storm-driven transport of foraminifers from the shelf to the upper slope, southern Middle Atlantic Bight

Abstract

Storms play an important role in the delivery of benthonic foraminifers to the continental slope, as observed in a study of foraminifer fluxes through the upper slope water column. The authors studied 30 sediment-trap samples with a 13-day average period from the 1988–1989 SEEP II experiment offshore from the Delmarva Peninsula. The traps were suspended at about 125 m water depth on a mooring in 400 m of water. Benthonic and planktonic foraminifers from 10-ml subsamples were measured, identified by taxa and growth stage, and counted. Number fluxes of benthonic foraminifers averaged 155 test/m 2/d during periods of relative calm during the spring and summer, when mass fluxes of aluminosilicates were also minimal. In contrast, number fluxes of benthonic foraminifers peaked during a mid-April 1988 storm and ranged from about 300 to 50,000 tests/m 2/d from mid-December 1988 to the end of April 1989, when mass fluxes of aluminosilicates also were highly elevated. Highest foraminifer fluxes (29,000 and 50,000 tests/m 2/d) coincided with a late February storm. Taxa observed included Bolivina, Nonionella, Trochammina, Rosalina, and other taxa typical of the continental shelf of this region. Number fluxes of planktonic foraminifers peaked during the spring and summer due to production. The peaks from 6000 to 11,000 tests/m 2/d were due to peaks in productivity of Globigerinita glutinata in early March and late April, Turborotalita quinqueloba in mid-July, and Globigerinita uvula and Globigerinoides ruber in latest September. Planktonic foraminifer fluxes did not crest during the mid-April or mid-December 1988 storms, but fluxes reached peaks of 38,000 and 41,000 tests/m 2/d in late February and early March 1989 when fluxes of benthonic foraminifers and aluminosilicate material also were highest. Storms dominated the delivery of both benthonic and planktonic foraminifers to the slope. The single storm in late February 1989 delivered more foraminifers through the water column to the slope (120 × 10 4 benthonic and 130 × 10 4 planktonic tests/m 2 in 32 days) than during all the preceding calm days in 1988 (1.9 × 104 benthonic and 72 × 104 planktonic tests/m 2 in 217 days). Mid-water advection of benthonic foraminifers from the continental shelf to the slope is an important mechanism of delivery that exceeds by an order of magnitude the numbers of planktonic foraminifers produced in slope waters during periods of relative calm weather.