Author: Hanneke Heida (Institut Ciències de la Terra Jaume Almera (CSIC), Barcelona – Spain)
Over the last century, global sea-levels have been rising at a steadily increasing pace, driven by melting glaciers and continent-based ice sheets and thermal expansion of ocean water. The current pace of the rise of sea level is estimated to be around 3.4-3.6 millimeters per year. Current sea levels are at a level 13-20 cm above those of the early 20th century, and although they were relatively stable during the preceding 2000 years (thought to be a key factor in the development of agricultural societies and therefore modern civilization), the earths sedimentary record shows clues to far more dramatic fluctuations.
Over geological time, two factors determine average global sea level: the volume of water (changing relatively rapidly over hundreds or thousands of years) and the size of the earths ocean basins, the depth and shape of which is changing over millions of years due to plate tectonic processes. The positions of the continents also control the surface area on which land ice can grow during a glaciation, and therefore its impact on sea levels, indicating that these long-term changes also affect the magnitude of the shorter timescale effects.
As illustrated in my last post, determining the origin of local changes in sea level and the contribution of the global average can be tricky even in the modern day, and sea-level is currently monitored by a sophisticated network of land-based gauges and satellites, just to filter out the effects of tidal and wave changes and local tectonic rise and fall. So how do we know what happened millions of years ago, and can we link changes in global sea-level (eustasy) to other events in the Earth’s history?
Methods available for determining eustatic changes include satellite measurements (last ~25 years), tide gauges (continuous 150-year record), and on longer timescales shoreline markers like fossil beaches, reefs and atolls, and the distribution of continental and marine sediments on continental margins.
Another regularly used proxy for the amount of water stored in ice is the ratio of oxygen isotopes 18 and 16. The lighter of the two is more easily evaporated from the ocean and more likely to get stored in ice during an ice age leaving the oceans enriched in the heavier isotope. This means that these stages are marked by a higher concentration of 18O isotopes in the shells of foraminifera in marine sediments globally. Carefully correcting for long-term changes in the sensitivity of this isotope ratio to other geological changes over time, it can be used to construct a record of glaciations and climate.
One of the first proposed mechanisms for the isolation of the Mediterranean from the Atlantic at the start of the MSC was ice growth on Antarctica, lowering sea level at the strait area connecting the oceans as the topography was being affected by the approach of Africa and Europe. This correlation was based on oxygen isotope records and the first attempts at dating the onset of the salinity crisis in ODP cores but the theory of global cooling triggering the onset of the MSC has remained very controversial with other age models for different stages of the MSC.
A lot of questions regarding global sea-level and the MSC remain. What was the impact of glaciations and eustatic variations during the stages of the MSC where the Atlantic and Mediterranean were still connected? Did they trigger small drops in sea-level and erosive stages, did such a phase perhaps cause increased erosion at the Gibraltar strait creating an opportunity for reflooding? And did the crisis affect the global sea-level by adding the Mediterranean waters to the global ocean during a stage of desiccation? The sedimentary record is sure to hold clues to these questions in the form of erosion surfaces, drowned reefs, fossil beaches and strange isotope excursions, hopefully guiding us to a better understanding of the Messinian Salinity Crisis.
Smithsonian Institution, https://ocean.si.edu/through-time/ancient-seas/sea-level-rise
Feynman, J., & Ruzmaikin, A. (2007). Climate stability and the development of agricultural societies. Climatic Change, 84(3–4), 295–311. https://doi.org/10.1007/s10584-007-9248-1
Kastens, K. A. (1992). Did glacio‐eustatic sea level drop trigger the Messinian salinity crisis? New evidence from Ocean Drilling Program Site 654 in the Tyrrhenian Sea. Paleoceanography, 7(3), 333–356. https://doi.org/10.1029/92PA00717
Lisiecki, L. E., & Raymo, M. E. (2005). A Pliocene-Pleistocene stack of 57 globally distributed benthic δ 18O records. Paleoceanography, 20(1), 1–17. https://doi.org/10.1029/2004PA001071
Miller, K. G., Kominz, M. A., Browning, J. V., Wright, J. D., Mountain, G. S., Katz, M. E., … Pekar, S. F. (2005). The Phanerozoic Record of Global Sea-Level Change. Science, 310, 1293–1298. Retrieved from http://www.jstor.org/stable/3843203