GEOCARB III: A revised model of atmospheric CO2 over phanerozoic time

Berner, RA Kothavala, Z

Ra. Berner et Z. Kothavala, GEOCARB III: A revised model of atmospheric CO2 over phanerozoic time, AM J SCI, 301(2), 2001, pp. 182-204

61

INGLESE

Article

Earth Sciences

AMERICAN JOURNAL OF SCIENCE

0002-9599 → ACNP

301

2

2001

182 - 204

ISI

0002-9599(200102)301:2<182:GIARMO>2.0.ZU;2-4

Revision of the GEOCARB model (Berner, 1991, 1994) for paleolevels of atmos pheric CO2, has been made with emphasis on factors affecting CO2 uptake by continental weathering. This includes: (1) new GCM (general circulation mod el) results: for the dependence of global mean surface temperature and runo ff on CO2, for both glaciated and non-glaciated periods, coupled with new r esults for the temperature response to changes hi solar radiation; (2) demo nstration that values for the weathering-uplift factor f(R)(t) based on Sr isotopes as was done in GEOCARB II are in general agreement with independen t values calculated from the abundance of terrigenous sediments as a measur e of global physical erosion rate over Phanerozoic time; (3) more accurate estimates of the timing and the quantitative effects on Ca-Mg silicate weat hering of the rise of large vascular plants on tile continents during the D evonian; (4) inclusion of the effects of changes in paleogeography alone (c onstant CO2 and solar radiation) on global mean land surface temperature as it affects tile rate of weathering; (5) consideration of the effects of vo lcanic weathering, both in subduction zones and on the seafloor (6) use of new data on the delta C-13 values for Phanerozoic limestones and organic ma tter; (7) consideration of the relative weathering enhancement by gymnosper ms versus angiosperms; (8) revision of paleo land area based on more recent data and use of this data, along with GCM-based paleo-runoff results, to c alculate global water discharge from the continents over time. Results show a similar overall pattern to those for GEOCARB II: very high C O2 values during the early Paleozoic, a large drop during the Devonian and Carboniferous, high values during the early Mesozoic, and a gradual decreas e from about 170 Ma to low values during the Cenozoic. However, the new res ults exhibit considerably higher CO2 values during the Mesozoic, and their downward trend with time agrees with the independent estimates of Ekart and others (1999). Sensitivity analysis shows that results for paleo-CO2 are e specially sensitive to: the effects of CO2 fertilization and temperature on the acceleration of plant-mediated chemical weathering; die quantitative e ffects of plants on mineral dissolution rate for constant temperature and C O2; the relative roles of angiosperms and gymnosperms in accelerating rock weathering; and the response of paleo-temperature to the global climate mod el used. This emphasizes the need for further study of the role of plants i n chemical weathering and the application of GCMs to study of paleo-CO2 and the long term carbon cycle.