Over the last 50 years, the terrestrial biosphere has absorbed ~25% of anthropogenic carbon emissions. Yet the behaviour of the terrestrial biosphere in the coming years is highly uncertain with even the sign of change (CO2 source vs CO2 sink) disputed. One of the main unquantified processes is the lateral transport of terrestrial organic carbon along the aquatic continuum from upland terrestrial ecosystems to the ocean. This process can act as a CO2 sink or CO2 source (Hilton and West, 2020). However, its importance in warmer climates remains a major gap in our understanding and a significant contributor to the overall uncertainty in predicting our future warm climate.
One way to test the behaviour of the Earth in warm climate states is to examine the geological record. In our recent study (Inglis et al., 2022; Palaeoceanography & Palaeoclimatology; 37), we used marine sediments deposited during the early Eocene Climatic Optimum (53.3–49.1 Ma) to quantify how much organic carbon is transferred from the terrestrial biosphere to the open ocean. The early Eocene Climatic Optimum is one of three high-CO2 climate states included in the recent Deep Time Model Intercomparison Project (DeepMIP; Hollis et al., 2019; Lunt et al., 2017) and is characterized by high temperatures (∼10°–16°C warmer than pre-industrial) and an intensified hydrological cycle (Inglis et al., 2020; Carmichael et al., 2016). As such, this interval can serve as a natural laboratory to understand how the terrestrial organic carbon cycle operates when it is significantly warmer and wetter than present.
A mock satellite image showing an ice-free and green Antarctic continent ~50 million years ago (Image credit: Dr Alan Kennedy). https://www.science.org/doi/10.1126/science.aad6284
In our study, we analysed Eocene-aged marine sediments from the East Antarctic shoreline (Integrated Ocean Drilling Program (IODP) Leg 318, Site U1356; Escutia et al., 2011; Figure 1). During the early Eocene, the Antarctic continent was ice-free and covered by a vast terrestrial organic carbon reservoir (DeConto et al., 2012; Figure 2) – there is even evidence for palm trees growing along the coastline! (Pross et al., 2012). Using biomolecules from plants and microbes, we determined how much organic matter was transferred from the Antarctic continent to the open ocean. We found evidence for increased terrestrial organic carbon burial in marine sediments during the early Eocene. This also coincides with enhanced moisture delivery to the East Antarctica margin, implying that wetter conditions promote terrestrial organic carbon burial. The transport and subsequent burial of terrestrial organic carbon in coastal marine sediments could have acted as a key CO2 sink during the early Eocene, but also during other climate intervals. This study highlights the importance of the terrestrial biosphere during past warm climates and its potential role as a negative feedback to stabilize Earth's surface temperature.
References:
Inglis, G.N., Toney, J., Zhu, J., Poulsen, C., Rohl, U., Pross, J., Cramwinckel, M., Krishnan, S., Pagani, M., Bijl, P.K and Bendle, J.B. Enhanced carbon export from the terrestrial biosphere during the early Eocene (2022) Paleoceanography & Paleoclimatology. 37. e2021PA004348
Escutia, C., Brinkhuis, H., Klaus, A., & the IODP Expedition 318 Scientists (2011). IODP Expedition 318; from greenhouse to icehouse at the Wilkes land Antarctic margin. Scientific Drilling, 12, 15–23. https://doi.org/10.5194/sd-12-15-2011
Inglis, G.N., Bragg, F., Burls, N., Cramwinckel, M.J., Evans, D., Foster, G.L., Huber, M., Lunt, D.J., Siler, N., Steinig, S., Tierney, J.E., Wilkinson, R., Anagnostou, E., de Boer, A.M., Dunkley Jones, T., Edgar, K., Hollis, C.J., Hutchinson, D.K and Pancost, R.D (2020) Global mean surface temperature and climate sensitivity of the EECO, PETM and latest Paleocene. Climate of the Past. 16. https://doi.org/10.5194/cp-16-1953-2020
DeConto, R. M., Galeotti, S., Pagani, M., Tracy, D., Schaefer, K., Zhang, T., et al. (2012). Past extreme warming events linked to massive carbon release from thawing permafrost. Nature, 484, 87–91. https://doi.org/10.1038/nature10929
Hollis, C. J., Dunkley Jones, et al (2019) The DeepMIP contribution to PMIP4: methodologies for selection, compilation and analysis of latest Paleocene and early Eocene climate proxy data, incorporating version 0.1 of the DeepMIP database. Geoscientific Model Development. 12. 3149-3206
Lunt, D.J., Huber, M., et al (2017) DeepMIP: experimental design for model simulations of the EECO, PETM, and pre-PETM. Geoscientific Model Development Discussion. 10. 889901
Carmichael, M. J., Lunt, D. J., Huber, M., Heinemann, M., Kiehl, J., LeGrande, A., et al. (2016). A model–model and data–model comparison for the early Eocene hydrological cycle. Climate of the Past, 12, 455–481. https://doi.org/10.5194/cp-12-455-2016
Hilton, R. G., & West, A. J. (2020). Mountains, erosion and the carbon cycle. Nature Reviews Earth & Environment, 1, 284–299. https://doi.org/10.1038/s43017-020-0058-6
Pross, J., Contreras, L., Bijl, P. K., Greenwood, D. R., Bohaty, S. M., Bohaty, S. M., et al. (2012). Persistent near-tropical warmth on the Antarctic continent during the early Eocene epoch. Nature, 488, 73–77. https://doi.org/10.1038/nature11300