How Hot Is Too Hot? Disentangling Mid‐Cretaceous Hothouse Paleoclimate From Diagenesis

• Published in: Paleoceanography and paleoclimatology Vol. 37; no. 12
• Main Authors: Fetrow, A. C., Snell, K. E., Di Fiori, R. V., Long, S. P., Bonde, J. W.
• Format: Journal Article
• Language: English
• Published: Hoboken Blackwell Publishing Ltd 01-12-2022
• Subjects: Canyons; Carbonate minerals; Carbonates; Catchment area; Climate change; Climatic conditions; clumped isotopes; Cretaceous; Datasets; Diagenesis; Fluids; Geochemistry; Geological history; Heterogeneity; History; hothouse; Isotopes; Lake deposits; Lakes; Low temperature; Minerals; Paleoclimate; Paleoclimatology; palustrine environments; Rock; Rocks; Seasons; Sedimentary facies; Stable isotopes; Summer temperatures; terrestrial carbonates; Warm seasons; Water temperature; Wetlands
• ISSN: 2572-4517; 2572-4525
• DOI: 10.1029/2022PA004517

The North American Newark Canyon Formation (NCF; ∼113–98 Ma) presents an opportunity to examine how terrestrial carbonate facies reflect different aspects of paleoclimate during one of the hottest periods of Earth's history. The lower NCF type section preserves heterogeneous palustrine facies and the upper NCF preserves lacustrine deposits. We combined carbonate facies analysis with δ13C, δ18O, and Δ47 data sets to assess which carbonate facies preserve stable isotope signals that are most representative of climatic conditions. Palustrine facies record the heterogeneity of the original wetland environment in which they formed. Using the pelmicrite facies that formed in deeper wetlands, we interpret a lower temperature zone (35–40°C) to reflect warm season water temperatures. In contrast, a mottled micrite facies which formed in shallower wetlands records hotter temperatures (36–68°C). These hotter temperatures reflect radiatively heated “bare‐skin” temperatures that occurred in a shallow depositional setting. The lower lacustrine unit has been secondarily altered by hydrothermal fluids while the upper lacustrine unit likely preserves primary temperatures and δ18Owater of catchment‐integrated precipitation. Resultantly, the palustrine pelmicrite and lacustrine micrite are the facies most likely to reflect ambient climate conditions, and therefore, are the best facies to use for paleoclimate interpretations. Average warm season water temperatures of 41.1 ± 3.6°C and 37.8 ± 2.5°C are preserved by the palustrine pelmicrite (∼113–112 Ma) and lacustrine micrite (∼112–103 Ma), respectively. These data support previous interpretations of the mid‐Cretaceous as a hothouse climate and demonstrate the importance of characterizing facies for identifying the data most representative of past climates.
Plain Language Summary
Considered a “supergreenhouse” world, the Cretaceous (145–65 Mya) was one of the hottest periods in geologic history. Understanding how environments respond to extreme global warmth provides insights that will help to mitigate the harmful effects of modern climate change. Geochemical signals, like stable isotopes, preserved in rocks allow us to reconstruct past climate conditions. We use stable isotope geochemistry on limestones (rocks made of carbonate minerals) to estimate average mid‐Cretaceous (∼110 Mya) summer temperatures in Nevada, USA. We examine limestones that formed in wetlands and lakes to assess which types record signals representative of the original landscape and which have values that were altered after the rock formed. We find that carbonates deposited in the shallowest parts of wetland environments record temperatures much hotter than air or deeper water temperatures due to extreme land surface heating, while deeper wetland limestones preserve temperatures more representative of ambient conditions. Average warm season water temperatures for wetland and lake carbonates are approximately 41 and 38°C, respectively, revealing hot conditions that support our understanding of the Cretaceous as a greenhouse world. This research provides an example of how to interpret useful climate information from complex carbonate data sets.
Key Points
Deeper palustrine and unaltered lacustrine carbonates record warm season water temperatures of approximately 41 and 38°C, respectively
High temperatures from well‐preserved shallower palustrine carbonates likely reflect radiative heating
Careful facies analysis is imperative for robust paleoclimate interpretation of heterogeneous terrestrial carbonate archives