Global change across the early Eocene and its warming events

Abstract

Under present-day anthropogenic climate change, the extent of future warming and its global distribution have become pressing scientific concerns. Crucial information for addressing these unknowns is captured within the geological record, offering a window into ancient climates under vastly different or fluctuating CO2 concentrations.

This work delves into the 'hothouse' conditions of the early Eocene (56–48 million years ago). This time was characterized by surface temperatures 10–14 °C higher than today and CO2 concentrations approximately three to five times higher than present, and ice sheets were presumably absent. During this period, many repetitive warming events (hyperthermals) occurred, driven by variations in Earth's orbit. Our knowledge of these hyperthermals was primarily limited to a few records of changing ocean bottom water conditions. Yet, these phases offer a great opportunity to study the effects of global warming.

In this work, early Eocene climate was reconstructed using sediment cores retrieved at three locations from tropics to poles: the equatorial Atlantic, the northern Negev, and the Arctic Ocean. The variability of surface ocean temperatures was reconstructed at these sites using temperature-sensitive molecular fossils, derived from marine archaea. Surface ocean conditions were further evaluated by analyzing assemblages of fossil organic walled dinoflagellate cysts.

A main result of this work is that a surface warming signal was observed during multiple hyperthermals and smaller orbital variations at all sites, confirming their global nature. During these variations, the polar regions experienced approximately twice the warming of the tropics, due to the effect of 'polar amplification'. In the absence of polar (sea)ice, this gives an interesting insight in the non-ice-related drivers of present-day polar amplification. Furthermore, the impact of the hyperthermals on tropical ocean surface ecosystems was investigated. Interestingly, no systematic biotic response was recorded during the smaller hyperthermals (below ~1.5 ºC surface warming). These finding contrasts previous work which documented significant biotic disruption during the larger hyperthermals and suggests a warming threshold for ocean surface biotic response. Finally, the effects of the warming intervals on the hydrological cycle were explored as well. These findings, amongst others, indicate that subtropical regions became drier during hyperthermals, while polar areas experienced wetter conditions.

In summary, this work presents new insights in the distribution and effects of warming on orbital scale during the early Eocene hothouse world.