Evolution of the Cenozoic carbon cycle: The roles of tectonics and CO₂ fertilization

Abstract

[1] Cenozoic carbon fluxes associated with rock weathering, sediment burial, and volcanic degassing are calculated from the mass balance equations coupling marine isotopic records of carbon (both organic and inorganic), strontium, and osmium. The result is confirmed by the good match between modeled carbonate sedimentation rates and carbonate sedimentation rates previously integrated from ocean basins worldwide. The coevolution between weathering and burial of carbonate suggests that marine carbonate accumulation was regulated mainly by the recycling of carbonate rocks, which mediated the bicarbonate ion concentration of the oceans. A reduction in CO 2 effusion to the atmosphere caused by reduced volcanic degassing from 52 to 15 Ma and tectonically enhanced organic rock exhumation since 15 Ma is also observed. These changes in CO 2 effusion are balanced by concomitant changes in CO 2 sequestration by silicate weathering and organic carbon burial. Importantly, we demonstrate a clear decoupling of modeled silicate weathering rates from global climate over the last $15 Ma. This observation is inconsistent with temperature-mediated mineral dissolution acting as the key mechanism facilitating the CO 2 silicate weathering feedback process. However, we instead observe a clear coupling (positive correlation) between modeled silicate weathering, organic carbon burial, and atmospheric CO 2 concentration. We suggest that CO 2 fertilization effects on terrestrial biomass productivity and plant weathering could have represented a major negative feedback process helping to balance atmospheric CO 2 , at least during the Cenozoic ice house periods.

A 20 million year record of planktic foraminiferal B/Ca ratios: Systematics and uncertainties in pCO2 reconstructions

“…However, total CO 2 degassing rates may not have changed significantly over the past 20 Ma (Li et al, 2009), as it is thought that spreading rates at mid-ocean ridges were relatively constant (Xu et al, 2006). Instead, it has been argued that there was a substantial increase in CO 2 sources since $15 Ma from the increasing denudation of organic-rich sedimentary cover caused either by tectonic uplift-forced erosion and/or increasing rates of glacial grinding (Li et al, 2009). If true, then given the well-constrained global cooling that has occurred over the past 15 Ma (Zachos et al, 2001), declining pCO 2 in the Mid-Miocene would likely be due to increasing the efficiency of CO 2 sinks.…”

“…Subsequently, during the Middle-Late Miocene ($14-10 Ma), there is evidence for a reduction in outgassing due to the cessation of flood basalt production (Hodell and Woodruff, 1994). However, total CO 2 degassing rates may not have changed significantly over the past 20 Ma (Li et al, 2009), as it is thought that spreading rates at mid-ocean ridges were relatively constant (Xu et al, 2006). Instead, it has been argued that there was a substantial increase in CO 2 sources since $15 Ma from the increasing denudation of organic-rich sedimentary cover caused either by tectonic uplift-forced erosion and/or increasing rates of glacial grinding (Li et al, 2009).…”

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

“…Hypothetically, mid-Miocene growth of the East Antarctic ice sheet may have lowered sea level (Zachos et al, 2001;Shackleton and Kennett, 1975;John et al, 2011), and exposed shallow marine carbonate platforms to chemical weathering (Li et al, 2009b). Reconstructions of riverine Sr isotopes during this time, though, do not show such endmember compositions (Galy et al, 1999;Oliver et al, 2003;Palmer and Edmond, 1992;Lear et al, 2003).…”

“…Although reconstructing the flux of any element is highly complex, and no unequivocal riverine flux record has ever been constructed, the river elemental fluxes have likely been highly variable over the past 40 Myr, due to changes in glaciation, orogeny, plant evolution, etc. (Li et al, 2009a;Li and West, 2014;Wanner et al, 2014). The Mg isotope ratio of rivers (present global flux-weighted mean ∼ −1.09 ‰, Tipper et al, 2006b) will dominantly depend on the ratio of carbonates (δ 26 Mg ∼ −5 to −1 ‰ ) to silicates (−0.25±0.1 ‰) in the catchment area Tipper et al, 2006aTipper et al, , b, 2008Tipper et al, , 2010Tipper et al, , 2012Li et al, 2010;Teng et al, 2010b).…”

Changing tectonic controls on the long‐term carbon cycle from Mesozoic to present

“…87 Sr/ 86 Sr of carbonates is a dynamic variable in the model, and reflects changes in the source and sink fractionations. Thus, over the model timeframe the 87 Sr/ 86 Sr of carbonates decreases due to burial of unradiogenic material to ∼0.7075 at the present day, which is close to current values for carbonate weathering (0.7077) [ Li et al ., ]. However, our reasonably successful reproduction of ocean 87 Sr/ 86 Sr represents only one way of achieving this.…”