Eric N. Powell

Taphonomic signature as a function of environmental process: shells and shell beds in a hurricane-influenced inlet on the Texas coast

Abstract Criteria for taphonomic processes, including dissolution, breakage, abrasion, size sorting and shell orientation, were statistically correlated against environmental parameters representing shell source, depositional environment and depth of burial, using samples obtained from San Luis Pass, a microtidal inlet on the Texas coast. Core intervals were differentiated by cluster analysis into shell-poor sands, slightly shelly sands, shell-rich sands and shell gravels based on their percent carbonate and percent shell gravel content. Boundaries between these sedimentary classes represent thresholds in physical processes forming the shell deposits. We propose that this classification scheme for shelly sediments may be useful for field descriptions in a variety of depositional and tectonic settings. The death assemblages were predominantly composed of whole and fragmental molluscan shells. Most shells show the effects of physical taphonomic processes such as shell breakage, edge rounding, surface abrasion and size sorting. A significant minority of the shells show evidence of minor chemical alteration (dissolution). Patterns of dissolution, edge rounding and surface abrasion differed significantly on shells in a mixed-faunal assemblage derived from different habitats (bay, inlet/beach, inner shelf and unrestricted). Thus, the habitat where a species lived, rather than its final depositional environment, controlled most aspects of its taphonomy. Only shell orientation and size-frequency distribution were significantly affected by final environment of deposition. The hypothesis that a discrete high energy (hurricane) “event” can imprint a unique taphonomic signature is, consequently, false. Although hurricanes physically winnow shell material, a single “event” merely concentrates shells with preexisting characteristics without further significant alteration. Depth of burial was poorly correlated with most biostratinomic effects, including dissolution. Significant dissolution must occur only near the sediment-water interface. Accordingly, quantified taphonomic characteristics might be useful in identifying the original habitats of species deposited in mixed accumulations; “taphofacies” reflect not only the final depositional process, but also taphonomic processes in the habitat of origin.

Time-averaging, taphonomy, and their impact on paleocommunity reconstruction: Death assemblages in Texas bays

The death assemblages in upper Copano Bay and upper Laguna Madre, Texas, were compared with the living communities as sampled over a two-year period in order to determine how taphonomy and time-averaging affect paleocommunity reconstruction. The living and death assemblages were compared using taxonomic composition, numerical abundance, diversity, biomass, and trophic and habitat proportions. Taxonomic composition, particularly adult taxonomic composition, and the biomass of the death assemblage more accurately reflect the characteristics of the living community than do numerical abundance and other community attributes based upon numerical abundance such as diversity. Taxonomic composition and biomass are less modified by taphonomy and time-averaging, because they are temporally more persistent characteristics of the community. Community and paleocommunity attributes are not equivalent, because the paleocommunity is described using time-averaged data. Ecological theory must be supplanted by paleoecological theory that takes into account this temporal component.

The rate of taphonomic loss in modern benthic habitats: how much of the potentially preservable community is preserved?

Abstract The post-mortem fate of shells added by natural mortality to modern death assemblages was studied to determine the importance of taphonomic loss during the initial stages of the formation of a fossil assemblage. Individuals are added to the living community in pulses, usually by larval settlement. These input pulses usually are followed by discrete input pulses into the death assemblage. The decay of these discrete input pulses in the death assemblage can be used to measure the rate of taphonomic loss as expressed by the half-life of a pulse. For instance, if a species has a half-life of 125 days and the input pulse begins with 500 individuals, then the number of shells remaining after one half-life (125 days) is 250 shells. At two sites on the Texas coast, the longest half-lives did not exceed one year for any of the 13 species which produced input pulses during the study period. Little net addition to the death assemblage could be detected in spite of the transitory addition of hundreds of shells as input pulses. These rapid decay rates show that death assemblages do not accumulate at the rate at which organisms die. Some species, such as the infaunal bivalve Tagelus plebeius , did not decay measurably. We predict that decay rates drop substantially with depth in the sediment so that individuals which die at depth or are rapidly reworked below the sediment surface have a greater chance of preservation. The final composition of death assemblages is probably little related to the actual numerical input from the potentially preservable component of the living community.

Distribution of Perkinsus marinus in gulf coast oyster populations

Prevalence (percent of oysters infected) ofPerkinsus marinus and infection intensity were measured in oysters from 49 sites in the Gulf of Mexico. Prevalence was less than 50% at only one site. Both prevalence and infection intensity were correlated with condition index, salinity, and a measure of local agricultural activity. The regional distribution ofP. marinus was patchy on spatial scales of 300 km or less and 1,500 km or more. Three regional foci of infection could be distinguished: the north central coast of Texas, central Louisiana west of the Mississippi River, and the southwestern coast of Florida. Lowest infection levels were recorded along the north central and northeastern Gulf, particularly east of the Mississippi River. The spatial distribution of infection varied with the salinity regime; however, other factors also explained part of the regional patterns observed. These included factors associated with man’s activities such as agricultural and industrial activity and the average annual temperature regime.

Movement of oxybiotic and thiobiotic meiofauna in response to changes in pore-water oxygen and sulfide gradients around macro-infaunal tubes

The effects of changes in vertical and horizontal microscale gradients of oxygen and sulfide on meiofaunal distributions were examined in laboratory microcosms. Specifically, the effect of tube abandonment and reestablishment by macro-infauna on the distribution of subsurface turbellarians, gnathostomulids and gastrotrichs was studied. Meiofauna responded rapidly (within 6 h) to changing sediment chemistry, consistently trying to reoccupy optimal habitat. Every subsurface taxon had a preferred suboptimal habitat which it occupied primarily during transit from deteriorating to newly established optimal habitat. Only during this time did the distribution of ecologically similar taxa overlap substantially. Changes in oxygen and sulfide gradients could explain most but not all of the response; food availability might also be important. Oxybios consistently chose oxic suboptimal microhabitat. Thus behaviorally, as well as biochemically and ecologically, thiobios represent a distinct group among the meiofauna.

Modeling oyster populations: V. Declining phytoplankton stocks and the population dynamics of American oyster (Crassostrea virginica) populations

Abstract Phytoplankton standing stocks have shown a steady decline in Galveston Bay, Texas over the last 20 years. Phytoplankton provides the primary food resource for oyster populations in Galveston Bay. We used a time-dependent population dynamics model of oyster populations to examine the impact of a decline in phytoplankton stocks on oyster populations. Simulations were run with two different types of mortality: winter mortality, assuming that the oyster fishery is the primary source of mortality; and summer mortality, assuming that predators and disease are the primary source of mortality. All simulations showed the same qualitative trends. Market-size adults disappear from the population in about 4 years, with an approximate 15% decline in food supply. Submarket-size adults maintained an increasing or steady population density for 10–14 years, after which the populations crashed to near extinction in 2–4 years. The proximate cause was a cessation in reproductive activity when food supply ceased to be sufficient to generate a fall spawn, an approximate 60% drop in food from current levels in Galveston Bay. The temporal sequence of mortality affected the outcome very little. The simulations suggest that populations decline rapidly in response to declining food supplies because a minimum food level is required to support a market-size population and a minimal reproductive activity. The simulations suggest that a reduction in market-size individuals is the primary early-warning signal of decreased food supply within the affected population and that this warning signal might easily be mistaken for overfishing. Proper management requires the monitoring of food supply and the use of a mathematical model to assess the importance of observed declines in population abundance. Unfortunately, once the fishery is affected, little time may remain before the termination of spawning and population extinction.

LOCAL VARIABILITY OF TAPHONOMIC ATTRIBUTES IN A PARAUTOCHTHONOUS ASSEMBLAGE: CAN TAPHONOMIC SIGNATURE DISTINGUISH A HETEROGENEOUS ENVIRONMENT?

ABsTRAcr-Taphofacies have been based on the likelihood that considerable variability exists in taphonomic processes between different environments and that this variability produces predictable variations in taphonomic signature between assemblages. Three stations above storm wave base that differed little in sediment texture and depth were sampled on the inner continental shelf of central Texas. Taphonomic analysis revealed subtle gradients in sediment grain size and water depth that would not be revealed by most other analyses. These gradients may exist over very small spatial scales, equivalent to those within a single extensive outcrop. Not all taphonomic attributes are equally likely to be preserved in the fossil record. Those varying with depth in our study area, such as fragmentation and articulation, are more likely to be preserved than those documenting changes in sediment texture, such as variation in the frequency of dissolution features on the shells. Nevertheless, siting and sampling protocols are important when characterizing a taphofacies because within-habitat variation is potentially as large as between-habitat variation. Description of the average taphofacies for an environment must include documentation of the variation in taphonomic attributes within the sampled area because few conservative taphonomic attributes exist. Fragments, even those that are unidentifiable, retain significant taphonomic information and should not be ignored. Careful sampling should permit the simultaneous description of general taphofacies as well as the detection of important but unsuspected gradients in the environment.

Changes in the free amino acid pool during environmental stress in the gill tissue of the oyster, Crassostrea virginica

Abstract 1. 1. Oysters were exposed for 2- and 5-day periods to increased salinity (26%.–38%.), anoxia, turbidity and drilling effluents. 2. 2. After two days, the FAA pool in the gill tissue of oysters exposed to 38%. salinity had elevated glycine, alanine and β-alanine levels; oysters exposed to anoxia showed elevated glycine and alanine and decreased aspartic acid levels. 3. 3. After 2 days, both oysters exposed to turbidity and to drilling effluents had increased cysteic acid levels. Glutamic acid and alanine levels were also elevated in oysters exposed to drilling effluents. 4. 4. After 5 days, glycine, alanine and β-alanine remained above control levels in oysters exposed to increased salinity whereas in those exposed to anoxia, turbidity and drilling effluents, a significant decrease in most amino acids occurred with the total FAA pool decreasing by 50%. 5. 5. The FAA pools response was unique for each stress studied suggesting that the FAA pool may prove to be a useful diagnostic tool for determining a posteriori the causative agent responsible for a given stress response.

Are molluscan maximum life spans determined by long-term cycles in benthic communities?

SummaryA review of the maximum longevity of bivalves and gastropods indicates that a greater than average number of life spans coincide with the periods of long-term cycles in marine communities. Apparently, long-term cycles exert an important influence on marine communities by affecting the life spans of constituent species. Gastropods and bivalves are affected differently, longevities being determined by some cycles more than others in each group. Overall, molluscan longevities tend to be slightly longer than the corresponding cycle suggesting that there is selection pressure for life spans slightly longer than the cycle controlling recruitment success and generational replacement.

Taphonomic signature of petroleum seep assemblages on the Louisiana upper continental slope; recognition of autochthonous shell beds in the fossil record

Chemoautotrophically-based benthic communities on the Louisiana continental slope are currently producing the only significant localized, autochthonous shell accumulation in the northern Gulf of Mexico shelf and slope region. These sites are well below storm wave base and are not subject to anthropogenic disturbance. Five distinctive biofacies are associated with petroleum seepage, dominated respectively by vestimentiferan tubeworms, lucinid, thyasirid and vesicomyid clams and mytilid mussels. The taphonomy of petroleum seeps includes dissolution as the most pervasive mode of shell alteration throughout all the biofacies