The impact of global environmental changes on exceptional preservation, extinction, and recovery in the early Jurassic (late Pliensbachian-Toarcian)

A major concern today is how modern marine communities will respond to rising levels of atmospheric carbon dioxide and climate change, and the fossil record can provide critical information about how organisms withstood similar climate crises in the past. During the Early Jurassic there were significant perturbations in environmental conditions around the globe, such as thermal stress, low oxygen levels, higher nutrient concentration, and acidification in some parts of the ocean. Of particular importance are the Pliensbachian/Toarcian boundary event (~184.2 million years ago) and the Toarcian Oceanic Anoxic Event (TOAE, ~183 million years ago); these events represent two of the most severe environmental perturbations of the Early Jurassic Epoch, leading to marine ecosystem disruption and biotic crises. It is thought that these changes led to multiple (or multi-phased) biotic crises, but they may have also enhanced exceptional fossil preservation (Lagerstätte preservation). This Ph.D. dissertation addresses how changing environmental conditions in the Early Jurassic affected the exceptional preservation, extinction, and recovery of organisms, specifically across the Pliensbachian/Toarcian boundary and during the Toarcian-Oceanic Anoxic Event (TOAE) or Jenkyns Event in the early Toarcian. One of the biggest challenges is that the fossil record is heavily biased towards the preservation of hard skeletons since soft tissues are rarely preserved; thus, much of the fossil record provides an incomplete picture of ancient ecosystems. To assess the conditions that were conducive to enhanced soft tissue preservation in the Early Jurassic, three exceptionally preserved fossil deposits were studied from different geographical locations (Tethys versus Panthalassa Oceans) and depositional settings (shallow water carbonates versus deeper water shales). Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) analyses showed phosphatization is the main preservation pathway across all three deposits and multiple fossil taxa. Phosphatization requires abundance of Phosphorous and Oxygen. Our results highlight that although the global anoxic conditions aided in soft tissue preservation, the fossils were preserved during brief periods of oxygenation. This is a paradigm shift from the longstanding beliefs about preservation mode and provides unique insights about how deoxygenation conditions could contribute to soft tissue preservation. To understand the extinction and recovery dynamics of Early Jurassic marine communities, we investigated the level-bottom community structure (e.g., brachiopods and bivalves) from multiple localities deposited in an onshore-offshore transect from the Central High Atlas Mountains of Morocco. A quantitative paleoecological analyses of 1724 macrofaunal samples show that none of the hypothesized environmental changes caused a significant loss in diversity among the shallow water carbonate communities; instead, there was a diversification and proliferation of both bivalves and brachiopods through the middle and late Toarcian. There was a temporary disappearance of fauna during the Pliensbachian/Toarcian boundary event and during the Jenkyns event due to the shift in facies from carbonates to siliciclastics; most taxa were, however, re-established once the carbonate factory recovered (middle Toarcian). Furthermore, there is no significant loss in functional groups over time, rather an increase in the diversity of functional group with increasing species richness. New occurrences of several taxa from the Moroccan basin are reported that are considered extinct in other basins, thereby expanding the ranges of these taxa. The persistence of non-reefal taxa in the same region where reef builders were suffering significant diversity losses, highlights the resilience of the level-bottom communities. In this dissertation, I posit that Morocco was a refugia for level-bottom communities. Although warmer temperatures could be impacting the community structure, none of the environmental stressors were sufficiently disastrous to cause an extinction event amongst the non-reef communities. The dissertation provides critical information about preservational pathways of soft tissue fossils in diverse facies during periods of global deoxygenation and provides a roadmap for discovering more lagerstätten during similar conditions. In addition, my dissertation is the first quantitative paleoecological study of level-bottom marine communities from Morocco, and therefore, expands the paleontological data we have from Early Jurassic shallow water carbonate sections. Community survival through moderate extinction events, such as the Early Jurassic events, are generally overlooked because they are not as catastrophic as the larger events. In-depth studies of the resilient macrofaunal communities could provide critical insights into community survival, addressing concerns about the conservation of extant marine communities in our changing environment today during the modern-day climate crises.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Geological Sciences

Grantor

The University of Texas at Austin

Year dc:date.issued

2024

Author dc:creator

• Sinha, Sinjini

Advisor dc:contributor.advisor

• Martindale , Rowan C.

Committee members dc:contributor.committeemember

• Kemp, Melissa
• Clarke, Julia A

• Environmental change
• Early Jurassic

Language dc:language.iso

English

Identifier URI

https://doi.org/10.26153/tsw/59703

OAI identifier oai:identifier

oai:repositories.lib.utexas.edu:2152/132363