A climate crisis in the Cretaceous
The layers of limestone and marl exposed in the hills around Mardin are rock; they are a high-fidelity record of a planetary crisis. These sediments, part of the Derdere and Karababa formations, were deposited on the floor of the ancient Tethys Ocean during the Cretaceous Period. Specifically, they document the Cenomanian-Turonian boundary, dated to about 93.9 million years ago. This period represents one of the hottest intervals of the last half-billion years, with tropical sea surface temperatures potentially reaching as high as 36°C.
This intense warmth was driven by a massive pulse of volcanic activity, likely linked to the formation of large igneous provinces like the Kerguelen or Caribbean LIPs. These eruptions vented enormous quantities of carbon dioxide into the atmosphere, triggering extreme greenhouse conditions. The warmer ocean waters stratified, and an accelerated water cycle washed huge amounts of nutrients from the continents into the sea. This surge in nutrients fueled immense blooms of plankton. When this organic matter died and sank, its decomposition consumed the oxygen in the seawater, leading to a worldwide ocean suffocation known as Oceanic Anoxic Event 2 (OAE2).
Reading the chemical echo
The Mardin limestone captures this ancient anoxic event through a distinct chemical fingerprint. The evidence lies in the isotopes of carbon. Organisms, from plankton to bacteria, preferentially use the lighter isotope, carbon-12 (¹²C), in their biological processes. During OAE2, vast quantities of this ¹²C-rich organic matter were buried in oxygen-starved sediments before they could decompose. This widespread burial effectively removed huge amounts of ¹²C from the ocean-atmosphere system.
The remaining carbon dissolved in the ocean became unusually enriched in the heavier isotope, carbon-13 (¹³C). When new layers of limestone formed from the carbonate shells of marine organisms, they locked in this skewed isotopic ratio. Geologists see this as a positive carbon isotope excursion (CIE) in the rock record—one of the largest disturbances in the global carbon cycle in the last 110 million years. In carbonate rocks from this period, the shift can be a positive excursion of 2 to 3 parts per thousand (‰), a clear signal of the massive carbon burial that occurred during the anoxic event. The dark, organic-rich shale layers, sometimes called "black shales," found within the sequence are the physical evidence of this buried organic matter. The entire anoxic event and its associated isotopic shift are estimated to have lasted for around 800,000 to 900,000 years.