Microstratigraphic Response of Molluscan Death Assemblages to Environmental Change
C. A. Ferguson
University of Cincinnati, Cincinnati, OH
Marine death assemblages reflect ecological and environmental signatures of depositional settings and display long-term compositional stability. Poorly understood, however, is the capacity of death assemblages to respond to environmental change on decadal time-scales or to preserve stratigraphic signals of altered conditions. This investigation examines the response of molluscan death assemblages in Florida Bay to an instance of environmental change, precipitated by nutrient pollution. The regionally dominant seagrass species, Thalassia testudinum, was outcompeted under nutrient enriched conditions by an opportunistic species, Halodule wrightii, which lacks morphological properties important for several molluscan inhabitants.
The objective of this study was to assess the preservation potential of environmental change among molluscan proxies, at the sediment surface and as a stratigraphically distinguishable signal in the sedimentary column. To achieve these ends, surficial push cores and deeper piston cores (up to 1.25 m) were collected at altered and unaltered sites. Surficial samples provide means of linking post-change death assemblages to current benthic conditions, to locate the stratigraphic transition point within and among core samples from Thalassia- to Halodule-derived death assemblages. Results indicate that: 1) the transition to a Halodule-dominated setting produced unique death assemblages compared to background conditions; and 2) molluscan death assemblages retain stratigraphic records of compositional change, reflecting variation in paleoenvironmental conditions over short time-scales, as the remains of subsequent generations accumulate over their predecessors. Thus, discrete lithological units may contain continuous stratigraphic records of paleoenvironmental variation detectable at centimeter-decimeter scales within sedimentary sequences, despite postmortem processes that may blur this signal.
AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas