Abstract
Not all boundaries, whether stratigraphical or geographical, are marked by species-level changes in community composition. For example, paleodata for some sites do not show readily discernible glacial-interglacial contrasts. Rather, the proportional abundances of species can vary subtly between glacials and interglacials. This paper presents a simple quantitative measure of assemblage turnover (assemblage turnover index, ATI) that uses changes in species' proportional abundances to identify intervals of community change. A second, functionally-related index (conditioned-on-boundary index, CoBI) identifies species contributions to the total assemblage turnover. With these measures we examine benthonic foraminiferal assemblages to assess glacial/interglacial contrasts at abyssal depths. Our results indicate that these measures, ATI and CoBI, have potential as sequence stratigraphic tools in abyssal depth deposits. Many peaks in the set of values of ATI coincide with terminations at the end of glaciations and delineate peak-bounded ATI intervals (PATIs) separated by boundaries that approximate to glacial terminations and to transgressions at neritic depths. These measures, however, can be used to evaluate the assemblage turnover and composition at any defined ecological or paleoecological boundary. The section used is from Ocean Drilling Program (OPD) Hole 994C, drilled on the Blake Ridge, offshore SE USA.
Highlights
Biostratigraphers historically have sought means to subdivide the sedimentary stratigraphic record as finely as possible
They were provided by the Ocean Drilling Program (ODP) that is sponsored by the U.S National Science Foundation (NSF) and participating countries
assemblage turnover index (ATI) was negatively correlated with max(pi) (r = –0.65, p,0.0001), which indicates a change in dominance across peaks in ATIs, and negatively correlated with d18O (r = –0.32, p,0.01)
Summary
Biostratigraphers historically have sought means to subdivide the sedimentary stratigraphic record as finely as possible. Thomas et al [16] examined benthonic foraminifera in two lower bathyal (,1700 m) and abyssal (,3500 m) piston cores spanning the last 45 ka in the northeastern Atlantic Ocean They found that Epistominella exigua (Brady) and Alabaminella weddellensis (Earland), which bloom opportunistically where a spring plankton bloom produces a pulse of phytodetritus, were rare during the last glacial maximum but abundant in the Holocene. Thomas et al [16] suggested that this reflects an enhanced organic carbon flux during glacials, rather than sluggish glacial bottom circulation leading to poorly oxygenated bottom water This may be related to the plankton multiplier effect proposed by Woods and Barkmann [18], in which a diminished greenhouse effect during glacials reduces radiative forcing of the ocean, increasing the depth of winter convection. Gaby and Sen Gupta [19] found glacial and postglacial assemblages of the abyssal
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