| Abstract: | Mid-Cretaceous CO2 estimates from proxy records and numerical carbon cycle models have a range of more than 4000 ppmv, which presents considerable uncertainty in understanding the possible causes of warmth for this interval. We examine the question of late Albian through Turonian radiative forcing from an inverse perspective: If general circulation models (GCMs) can be used to accurately predict surface temperatures and mid-Cretaceous surface and ocean temperatures can be reasonably estimated from proxy data, then GCMs that reproduce mid-Cretaceous temperature estimates can be used to define plausible greenhouse gas concentrations for the mid-Cretaceous. Upper ocean water temperatures are estimated from planktonic  data and compared against temperatures predicted by mid-Cretaceous atmospheric model experiments in which CO2 is varied from 500 to 7500 ppmv. In the GCM, reduced solar luminosity and possible increased CH4 are also taken into consideration. The results indicate that 4500-5000 ppmv CO2 is required to match the warmest upper ocean temperatures inferred from of well-preserved planktonic foraminifera, excluding late Turonian data from ODP Site 511. Approximately 900 ppmv CO2 produces a good match between the model and minimum tropical temperatures estimates for the mid-Cretaceous. An ocean model forced by these two extremes in surface conditions brackets nearly all available bottom water temperature estimates for this interval.
When very depleted (as light as -4.6 per mil PDB) late Turonian planktonic values from Site 511 (Huber et al., 1995; Bice et al., in prep.) are included in the model-data comparison, then 6500-7500 ppmv CO2 is required to produce a match to upper ocean temperatures. In the ocean model, this surface forcing produces warmer bottom water temperatures than are indicated by any existing mid-Cretaceous benthic data. The meaning of the Site 511 late Turonian data is currently unresolved.
Our results indicate that the models support nearly the entire range of mid-Cretaceous CO2 proxy reconstructions. This suggests that, rather than contradicting one another, proxy CO2 techniques with high temporal resolution may capture true variability in CO2 concentrations and that mid-Cretaceous atmospheric CO2 varied by several thousand ppmv on timescales of 2 million years or less. The ocean model suggests plausible buoyancy forcing conditions responsible for changes in the sites of deep water formation in the mid-Cretaceous. The dominant change is from deep water formation in the high latitude region of one hemisphere to the other hemisphere in response to warmer/cooler surface temperatures and an increased/decreased hydrologic cycle.
Huber, B. T., D. A. Hodell and C. P. Hamilton, Middle-Late Cretaceous climate of the southern high latitudes: Stable isotopic evidence for minimal equator-to-pole thermal gradients, GSA Bull., 107, 1164-1191, 1995. |