Distribution of Modern Benthic Foraminifera of McMurdo Sound, Antarctica

<p>This thesis presents the results of a study of benthic foraminifera from McMurdo Sound, Antarctica. The sound is 50 km across and more than 900 m deep, and is ice-covered for at least 9 months of the year. However, salinity and temperature of the bottom waters are constant (35 per mil and -1.8 degrees C). Sea floor sediment is mainly fine sand and mud with a little ice-rafted gravel. The aim of the study was to document the distribution of living and dead foraminifera and to determine the factor(s) controlling it. The twenty-six sites in water from 76 to 856m deep were sampled by gravity corer and grab, and nearly 40,000 specimens (2334 living and 36,875 dead) were identified. Three present day assemblages can be recognised: 1. Shallow open water assemblage (SWA): Trochammina glabra, Cribrostomoides jeffreysii, Trifarina earlandi, Ehrenbergina glabra, Fursenkoina earlandi and Globocassidulina crassa. 2. Deep open water assemblage (DWA): Reophax pilulifer, Reophax subdentaliniformis, Portotrochammina antarctica, Textularia antarctica and Miliammina arenacea. 3. Harbour/enclosed basin assemblage (HA): Reophax subdentaliniformis, Portotrochammina antarctica, Textularia antarctica, Fursenkoina earlandi and Globocassidulina crassa. The composition of the assemblages is controlled largely by calcium carbonate compensation depth (CCD). Calcareous species are abundant and varied (84 calcareous species) in the SWA above 620m, but are virtually absent from the DWA, which is found in deeper water. The dominance of agglutinated foraminifera in the HA indicates an even shallower CCD (about 270m) in restricted coastal settings. Death assemblages have a similar species diversity to corresponding life assemblages and are reasonably representative of them, except for the 200m zone above the offshore CCD, where death assemblages are depleted in calcareous taxa. The diversity of the agglutinated component of each assemblage remains nearly constant in all habitats and at all water depths, even though shallow water samples include a range of calcareous species. Thus competition from calcareous species appears not to be a stress factor for agglutinated species, which are considered to have reached the limit of their evolutionary potential in these waters.</p>

Distribution of Modern Benthic Foraminifera of McMurdo Sound, Antarctica

<p>This thesis presents the results of a study of benthic foraminifera from McMurdo Sound, Antarctica. The sound is 50 km across and more than 900 m deep, and is ice-covered for at least 9 months of the year. However, salinity and temperature of the bottom waters are constant (35 per mil and -1.8 degrees C). Sea floor sediment is mainly fine sand and mud with a little ice-rafted gravel. The aim of the study was to document the distribution of living and dead foraminifera and to determine the factor(s) controlling it. The twenty-six sites in water from 76 to 856m deep were sampled by gravity corer and grab, and nearly 40,000 specimens (2334 living and 36,875 dead) were identified. Three present day assemblages can be recognised: 1. Shallow open water assemblage (SWA): Trochammina glabra, Cribrostomoides jeffreysii, Trifarina earlandi, Ehrenbergina glabra, Fursenkoina earlandi and Globocassidulina crassa. 2. Deep open water assemblage (DWA): Reophax pilulifer, Reophax subdentaliniformis, Portotrochammina antarctica, Textularia antarctica and Miliammina arenacea. 3. Harbour/enclosed basin assemblage (HA): Reophax subdentaliniformis, Portotrochammina antarctica, Textularia antarctica, Fursenkoina earlandi and Globocassidulina crassa. The composition of the assemblages is controlled largely by calcium carbonate compensation depth (CCD). Calcareous species are abundant and varied (84 calcareous species) in the SWA above 620m, but are virtually absent from the DWA, which is found in deeper water. The dominance of agglutinated foraminifera in the HA indicates an even shallower CCD (about 270m) in restricted coastal settings. Death assemblages have a similar species diversity to corresponding life assemblages and are reasonably representative of them, except for the 200m zone above the offshore CCD, where death assemblages are depleted in calcareous taxa. The diversity of the agglutinated component of each assemblage remains nearly constant in all habitats and at all water depths, even though shallow water samples include a range of calcareous species. Thus competition from calcareous species appears not to be a stress factor for agglutinated species, which are considered to have reached the limit of their evolutionary potential in these waters.</p>

Testing the effects of durability (shell thickness) on species over-representation in death assemblages, a taphonomic baseline for conservation paleobiology

&lt;p&gt;Differences in the taxonomic or functional composition of living and death assemblages is a key means of identifying the magnitude and drivers of past ecological changes in conservation paleobiology, especially when assessing the effects of anthropogenic impacts. However, such live-dead differences in species abundances can arise not only from ecological (stochastic or deterministic) changes in abundance over the duration of time averaging but also from interspecific differences in the postmortem durability of skeletal remains or from the lifespan of the individuals. Here, we attempt to directly incorporate the effects of durability on species abundances in death assemblages by modeling dead abundance as a function of species&amp;#8217; durability traits and using abundances in living assemblages as a prior. Species inferred to be negatively affected by anthropogenic impacts should be over-represented in death assemblages relative to their abundance in death assemblages predicted by the durability model (rather than just relative to their abundance in living assemblages). Using species-level durability trait data for bivalves (shell size, thickness, mineralogy, shell organic content, and life habit) from the southern California shelf, we find that, among these traits, valve thickness correlates consistently positively and at multiple spatial scales with the log of the dead:live ratio of species abundances, and accounts for ~20-30% of live-dead mismatch. Using this benchmark for the discordance that might be taphonomic in origin, we confirm that the over-representation of epifaunal suspension-feeders and siphonate deposit-feeders in death assemblages of the southern California shelf owes in fact to their ecological decline in recent centuries, even when accounting for their greater durability.&lt;/p&gt;

Addressing challenges in biodiversity conservation with fish otolith death assemblages: the state-of-the-art

&lt;p&gt;Fish otoliths are incrementally growing aragonitic elements found in the inner ear of most fishes. They have species-specific morphology that enable species level identification and they are excellent high-resolution recorders of ambient water conditions, enabling the reconstruction of past fish faunas and their environment. Although they have been studied as fossils for almost 150 years, and they are very useful tools for tracking lifestyle and population changes in modern fishes, otolith death assemblages recovered from sea bottom sediments have been studied only much more recently. Still, these fish remains can provide valuable insight into past fish faunas before most anthropogenic impacts, such as climate warming, habitat modification and biological invasions. Here, we present an overview of research done until now on otolith death assemblages highlighting their applications for marine conservation.&lt;/p&gt;

Taphonomic information from the modern vertebrate death assemblage of Doñana National Park, Spain

Modern death assemblages provide insights about the early stages of fossilization and useful ecological information about the species inhabiting the ecosystem. We present the results of taphonomic monitoring of modern vertebrate carcasses and bones from Doñana National Park, a Mediterranean coastal ecosystem in Andalusia, Spain. Ten different habitats were surveyed. Half of them occur in active depositional environments (marshland, lake margin, river margin, beach and dunes). Most of the skeletal remains belong to land mammals larger than 5 kg in body weight (mainly wild and feral ungulates). Overall, the Doñana bone assemblage shows good preservation with little damage to the bones, partly as a consequence of the low predator pressure on large vertebrates. Assemblages from active depositional habitats differ significantly from other habitats in terms of the higher incidence of breakage and chewing marks on bones in the latter, which result from scavenging, mainly by wild boar and red fox. The lake-margin and river-margin death assemblages have high concentrations of well preserved bones that are undergoing burial and offer the greatest potential to produce fossil assemblages. The spatial distribution of species in the Doñana death assemblage generally reflects the preferred habitats of the species in life. Meadows seem to be a preferred winter habitat for male deer, given the high number of shed antlers recorded there. This study is further proof that taphonomy can provide powerful insights to better understand the ecology of modern species and to infer past and future scenarios for the fossil record.

Live, dead, and fossil mollusks in Florida freshwater springs and spring-fed rivers: Taphonomic pathways and the formation of multisourced, time-averaged death assemblages

Taphonomic processes are informative about the magnitude and timing of paleoecological changes but remain poorly understood with respect to freshwater invertebrates in spring-fed rivers and streams. We compared taphonomic alteration among freshwater gastropods in live, dead (surficial shell accumulations), and fossil (late Pleistocene–early Holocene in situ sediments) assemblages from two Florida spring-fed systems, the Wakulla and Silver/Ocklawaha Rivers. We assessed taphonomy of two gastropod species: the native Elimia floridensis (n = 2504) and introduced Melanoides tuberculata (n = 168). We quantified seven taphonomic attributes (aperture condition, color, fragmentation, abrasion, juvenile spire condition, dissolution, and exterior luster) and combined those attributes into a total taphonomic score (TT). Fossil E. floridensis specimens exhibited the greatest degradation (highest TT scores), whereas live specimens of both species were least degraded. Specimens of E. floridensis from death assemblages were less altered than fossil specimens of the same species. Within death assemblages, specimens of M. tuberculata were significantly less altered than specimens of E. floridensis, but highly degraded specimens dominated in both species. Radiocarbon dates on fossils clustered between 9792 and 7087 cal BP, whereas death assemblage ages ranged from 10,692 to 1173 cal BP. Possible explanations for the observed taphonomic patterns include: (1) rapid taphonomic shell alteration, (2) prolonged near-surface exposure to moderate alteration rates, and/or (3) introduction of reworked fossil shells into surficial assemblages. Combined radiocarbon dates and taphonomic analyses suggest that all these processes may have played a role in death assemblage formation. In these fluvial settings, shell accumulations develop as a complex mixture of specimens derived from multiple sources and characterized by multimillennial time-averaging. These findings suggest that, when available, fossil assemblages may be more appropriate than death assemblages for assessing preindustrial faunal associations and recent anthropogenic changes in freshwater ecosystems.