Cretaceous Period's Cold Climate Altered by Volcanic Warm Spurts

Paleo-climate ambiguities have often caused a stir in the scientific community, particularly in relation to the early Cretaceous period. One of the most contentious issues is whether Antarctic ice sheets existed during this era. To address these uncertainties, we conducted rigorous testing by reconstructing CO2 concentrations from the C3 plant proxy and sea level curve using the δ18O proxy of marine organisms during the aptian stage. This time interval offered a wealth of data for analysis within an intricate temporal framework. 
 
We then reconstructed sea levels for the early Cretaceous using similar methods due to a lack of reliable CO2 proxy data. These curves were juxtaposed with Oceanic Anoxic Events (OAEs) and Large Igneous Provinces (LIPs), potential causal mechanisms influencing climate patterns. Our results revealed low atmospheric CO2 concentrations and sea levels coupled with glacial indicators such as tillites and glendonites. These findings substantiate the presence of small to large ice sheets in southern polar regions throughout much of the early Cretaceous, which temporarily melted due to episodic volcanism. 
 
To validate our hypothesis that atmospheric CO concentrations were sufficiently low to sustain glaciers during this period, we utilized both a plant-based carbon dioxide (CO) concentration indicator known as 'C Plant Proxy' and another method involving isotopic excursion between atmospheric and terrestrial environments (CIE-CIE). The first approach was based on experimental work demonstrating an inverse link between atmospheric CO concentration and δC values in plants, while Δ(CIE-CIE) was derived from mass balance considerations. 
 
The reconstruction process required comparable δC values for both atmospheric CO2 and plant substrate at corresponding periods; any violation could result in inaccurate outcomes due to variations among species or moisture availability differences over time. Some researchers had previously suggested that plants during the Aptian might not have responded effectively to fluctuations in CO concentration owing to relatively high organic δC values; hence, we decided upon reconstructing maximum baseline carbon dioxide (CO2) concentrations prior to two carbon isotope excursions during the Aptian using Δ(CIE-CIE). 
 
For creating the curve that represented atmospheric CO concentration, we leveraged carbonate equilibrium reactions on bulk carbonate δC and δO values derived from Deep Sea Drilling Project (DSDP) 463 in the South Pacific. The initial age model for this core was developed based on ammonite zonation and stage boundaries, where Aptian stratigraphic records were thicker than most other localities. 
 
To account for vegetation-related factors, we compiled previously published data about gymnosperm wood from different regions, including the Isle of Wight, Arctic Svalbard, and Japan. The geochronology of this record was then applied to assign a value to each of those woods, δC. 
 
Our analysis showed similar CO concentrations across these three locations in terms of magnitude as well as trend. There were visible spikes in carbon dioxide median concentrations at certain intervals, which suggest an increase in greenhouse gases during those periods, possibly due to volcanic activities or oceanic anoxic events. 
 
We also made simultaneous calculations for Δ(CIE-CIE) through mass balance constraints before OAEs, which suggested baseline CO2 levels could have been approximately 482 ppm prior to OAE 1a and around 215 ppm prior to end-Aptian OAE, aligning with our earlier findings generated by the C plant proxy approach. 
 
Despite uncertainties associated with quantitative CO estimates via both methods used, it appears likely that baseline Aptian carbon dioxide concentration lay below the simulated threshold required for polar ice sheets, further corroborated by the presence of tillites and glendonites during the early-late Aptian period, suggesting cold climate conditions prevailed back then. 
 
Our study revealed short-lived spurts in atmospheric CO2 concentration amidst predominantly low levels, generally under 840 ppm. During specific oceanic anoxic events like Selli or Noir, high spikes indicated temporary warming phases when ice sheets were likely not present on Antarctica. However, these bursts of greenhouse gases seem to have been regulated by organic carbon abundance, which likely kept atmospheric CO2 levels below 840 ppm. 
 
We also examined sea level fluctuations during the early Cretaceous period, assuming the presence of ice sheets would have caused sea level changes due to a combination of orbital and tectonic forcing. Our reconstruction based on δO values from belemnites for the Aptian study area and benthic foraminifera from the Vocontian Basin showed that the early Cretaceous sea level was mainly below the ice-free datum, indicating the possibility of persistent Antarctic ice sheets despite rapid fluctuations in water levels. 
 
In conclusion, our findings suggest that, contrary to previous beliefs about warm climate conditions during the early Cretaceous era, there's substantial evidence supporting the existence of fluctuating but persistent polar ice sheets indicating a colder climatic state than previously assumed.