Eutrophication, microbial-sulfate reduction and mass extinctions.

Area of Science:

• Paleontology
• Geochemistry
• Oceanography

Background:

• Earth has experienced five major extinction events since the Cambrian period.
• Three of these, the Late Devonian, end-Permian, and end-Triassic, show links between marine biodiversity loss and anoxic, sulfidic waters.
• Geochemical and sedimentary data suggest these extinctions correlate with abrupt climate warming and increased terrestrial weathering.

Purpose of the Study:

• To investigate the role of Earth's intrinsic biogeochemical cycles, specifically sulfur and carbon, in triggering mass biodiversity loss.
• To propose a mechanism linking climate warming, eutrophication, and anoxia to marine extinctions.
• To assess the relevance of these extinction mechanisms to modern oceanic environments and future climate change.

Main Methods:

• Analysis of geochemical and sedimentary evidence from past extinction events.
• Modeling the feedback loops between climate warming, continental weathering, eutrophication, and ocean anoxia.
• Examining plankton community structure shifts from eukaryote-dominated to bacteria-dominated food webs.

Main Results:

• Evidence suggests that climate warming-induced eutrophication expands oxygen minimum zones (OMZs).
• Increased microbial sulfate reduction in OMZs leads to sulfidic conditions.
• A shift in plankton communities, favoring high-biomass bacteria over diverse eukaryotes, acts as a catalyst for extinction.

Conclusions:

• Mass marine extinctions can be triggered by intrinsic Earth system mechanisms, particularly the sulfur and carbon cycles, leading to anoxia.
• The proposed anoxia-extinction scenario contrasts with the productivity collapse model for the end-Cretaceous extinction.
• These findings are highly relevant to understanding and predicting the impact of future greenhouse-driven climate change on oceanic dead zones.