Abstract
The Pliensbachian-Toarcian (Early Jurassic) fossil record is an archive of natural data of benthic community response to global warming and marine long-term hypoxia and anoxia. In the early Toarcian mean temperatures increased by the same order of magnitude as that predicted for the near future; laminated, organic-rich, black shales were deposited in many shallow water epicontinental basins; and a biotic crisis occurred in the marine realm, with the extinction of approximately 5% of families and 26% of genera. High-resolution quantitative abundance data of benthic invertebrates were collected from the Cleveland Basin (North Yorkshire, UK), and analysed with multivariate statistical methods to detect how the fauna responded to environmental changes during the early Toarcian. Twelve biofacies were identified. Their changes through time closely resemble the pattern of faunal degradation and recovery observed in modern habitats affected by anoxia. All four successional stages of community structure recorded in modern studies are recognised in the fossil data (i.e. Stage III: climax; II: transitional; I: pioneer; 0: highly disturbed). Two main faunal turnover events occurred: (i) at the onset of anoxia, with the extinction of most benthic species and the survival of a few adapted to thrive in low-oxygen conditions (Stages I to 0) and (ii) in the recovery, when newly evolved species colonized the re-oxygenated soft sediments and the path of recovery did not retrace of pattern of ecological degradation (Stages I to II). The ordination of samples coupled with sedimentological and palaeotemperature proxy data indicate that the onset of anoxia and the extinction horizon coincide with both a rise in temperature and sea level. Our study of how faunal associations co-vary with long and short term sea level and temperature changes has implications for predicting the long-term effects of “dead zones” in modern oceans.
Highlights
The expansion of oxygen minimum ‘‘dead zones’’ in modern oceans is at the top of the list of emerging environmental challenges [1]
In particular we aim to assess (i) how alpha and beta diversity changed through the studied interval; (ii) how benthic communities responded to sea level, oxygen concentrations and temperature variations; and (iii) how this fossil event compares to the recorded patterns of faunal succession in modern communities affected by hypoxic and anoxic conditions
(i) Analyses of alpha and beta diversity demonstrate that a net decrease in diversity coupled with high faunal turnover characterizes the extinction horizon at the top of the Grey Shale Member
Summary
The expansion of oxygen minimum ‘‘dead zones’’ in modern oceans is at the top of the list of emerging environmental challenges [1]. General circulation models predict that climate change will directly deplete oceanic dissolved oxygen levels by increasing stratification and warming, as well as indirectly by causing changes in rainfall patterns, nutrient run-off and shelf eutrophication; all of which will increase the areas affected by hypoxia (dissolved oxygen #2 ml/l, [2]) and anoxia [3,4,5]. Hypoxia can affect coastal areas seasonally, periodically or inter-annually [3], and depending upon its duration and severity, it may take years or even decades for recovery of the original community composition [7]. The impact of lowoxygen conditions on ecosystems is relatively well understood through field and experimental studies, the processes of recovery are still poorly understood and the experimental assessment of hypoxia remains challenging [7]. The pattern of species reappearance post-event may follow a different trajectory to that of species loss, resulting in a hysteresis-like response of biodiversity to the alleviation of hypoxia [3]
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