https://phys.org/news/2018-05-ancient-fish-chicxulub-asteroid-planet.html

The fish remains were sifted from sediment samples collected at a site in El Kef, Tunisia. During the time before and long after the asteroid strike, the area was covered by the Tethys Sea. The researchers looked at oxygen ratios in the fish remains as a means of determining the temperature of the water at the time that the fish died. Collecting samples from different layers allowed for building a temperature timeline that began before the asteroid strike and lasting hundreds of thousands of years thereafter. In looking at their timeline the group found that sea temperatures had risen approximately 5°C not long after the asteroid struck and had stayed at that temperature for approximately 100,000 years.

The researchers suggest the strike by the asteroid very likely released a lot of carbon dioxide into the atmosphere because the ground area where it struck was rich in carbonates. The strike very likely would have also ignited large long-burning forest fires which would have also released a lot of carbon into the air. The evidence suggests that the cooling after the impact was short-lived as massive amounts of carbon dioxide were released into the atmosphere setting off global warming.