Understanding long-term carbon cycle trends: The late Paleocene through the early Eocene

Abstract

[1] The late Paleocene to the early Eocene ( 58-52 Ma) was marked by significant changes in global climate and carbon cycling. The evidence for these changes includes stable isotope records that reveal prominent decreases in ı 18 O and ı 13 C, suggesting a rise in Earth's surface temperature ( 4 ı C) and a drop in net carbon output from the ocean and atmosphere. Concurrently, deep-sea carbonate records at several sites indicate a deepening of the calcite compensation depth (CCD). Here we investigate possible causes (e.g., increased volcanic degassing or decreased net organic burial) for these observations, but from a new perspective. The basic model employed is a modified version of GEOCARB III. However, we have coupled this well-known geochemical model to LOSCAR (Long-term Ocean-atmosphere Sediment CArbon cycle Reservoir model), which enables simulation of seawater carbonate chemistry, the CCD, and ocean ı 13 C. We have also added a capacitor, in this case represented by gas hydrates, that can store and release 13 C-depleted carbon to and from the shallow geosphere over millions of years. We further consider accurate input data (e.g., ı 13 C of carbonate) on a currently accepted timescale that spans an interval much longer than the perturbation. Several different scenarios are investigated with the goal of consistency amongst inferred changes in temperature, the CCD, and surface ocean and deep ocean ı 13 C. The results strongly suggest that a decrease in net organic carbon burial drove carbon cycle changes during the late Paleocene and early Eocene, although an increase in volcanic activity might have contributed. Importantly, a drop in net organic carbon burial may represent increased oxidation of previously deposited organic carbon, such as stored in peat or gas hydrates. The model successfully recreates trends in Earth surface warming, as inferred from ı 18 O records, the CCD, and ı 13 C. At the moment, however, our coupled modeling effort cannot reproduce the magnitude of change in all these records collectively. Similar problems have arisen in simulations of short-term hyperthermal events during the early Paleogene (Paleocene-Eocene Thermal Maximum), suggesting one or more basic issues with data interpretation or geochemical modeling remain.

Did high Neo-Tethys subduction rates contribute to early Cenozoic warming?

“…Carbonates also indicate that from ∼ 58.0 to 52.5 Ma this warming was characterized by a 2 per mil negative shift in marine and terrestrial δ 13 C, referred to as the Late Paleocene-Early Eocene (LPEE) by Komar et al (2013). This drop in δ 13 C suggests an additional source of depleted CO 2 (i.e enriched in 12 C) or/and decreased net organic carbon burial (Hilting et al, 2008;Komar et al 2013). In contrast, despite warm temperature, the EECO was associated with a rise in δ 13 C (Cramer et al, 2009), indicative of the addition of heavy CO 2 or/and alternatively by increased net organic carbon burial (e.g., Komar et al, 2013).…”

“…Because conventional carbon cycle models compute important weathering rates at that time, they fail to reproduce this rise in temperature and atmospheric CO 2 without the addition of excess CO 2 compared to background CO 2 volcanic degassing rates (4-10 × 10 18 molCO 2 Ma −1 at present; Berner, 2004) (Lefebvre et al, 2013;Van der Meer et al, 2014). Carbonates also indicate that from ∼ 58.0 to 52.5 Ma this warming was characterized by a 2 per mil negative shift in marine and terrestrial δ 13 C, referred to as the Late Paleocene-Early Eocene (LPEE) by Komar et al (2013). This drop in δ 13 C suggests an additional source of depleted CO 2 (i.e enriched in 12 C) or/and decreased net organic carbon burial (Hilting et al, 2008;Komar et al 2013).…”

“…This drop in δ 13 C suggests an additional source of depleted CO 2 (i.e enriched in 12 C) or/and decreased net organic carbon burial (Hilting et al, 2008;Komar et al 2013). In contrast, despite warm temperature, the EECO was associated with a rise in δ 13 C (Cramer et al, 2009), indicative of the addition of heavy CO 2 or/and alternatively by increased net organic carbon burial (e.g., Komar et al, 2013). Various origins of excess CO 2 have been proposed for both periods of the early Cenozoic.…”

“…Most invoke the activity of large igneous provinces such as the North Atlantic Igneous Province (NAIP), since a mantellic source of CO 2 (δ 13 CO 2 ranging from −3 to −10 ‰) may be compatible with carbon isotope proxies for most of the period of warming (see Reagan et al 2013 and references therein). Alternatively, Beck et al (1995), Hilting et al (2008) and Komar et al (2013) proposed that large amounts of low-δ 13 C organic carbon were being stored in carbon capacitors separate from the oceanatmosphere-biosphere (e.g., peat, gas hydrates, permafrost) during the Paleocene. They were then massively released during the LPEE warming and progressively vanished during the EECO (Komar et al, 2013).…”

“…Alternatively, Beck et al (1995), Hilting et al (2008) and Komar et al (2013) proposed that large amounts of low-δ 13 C organic carbon were being stored in carbon capacitors separate from the oceanatmosphere-biosphere (e.g., peat, gas hydrates, permafrost) during the Paleocene. They were then massively released during the LPEE warming and progressively vanished during the EECO (Komar et al, 2013). Finally, among several other hypotheses, it was suggested that Neo-Tethys closure may have strongly controlled Cretaceous and early Cenozoic climates, up to the EECO, through the subduction of tropical pelagic carbonates (δ 13 C ∼ 0 ‰) under the Asian plate and their recycling as CO 2 at arc volcanoes (Edmond and Huh, 2003;Kent and Muttoni, 2008;Johnston et al, 2011).…”