Geologic Records of High Sea Levels Completed

By Geosciences and Environmental Change Science Center July 14, 2017

Coral reef marine terraces

Talafofo Beach, Saipan

Coral/algal heads in an ancient reef

Oahu, Hawaii

Fossil coral reefs indicate earlier “high water mark”

Windley Key quarry, Florida

This project studies past high sea levels on coastlines that preserve fossil coral reefs or marine terraces. We ascertain the magnitudes of sea-level high stands by field mapping, stratigraphic measurements, and precise elevation measurements. Geochronology is accomplished by radiocarbon dating of mollusks (for Holocene-to-last-glacial deposits), uranium-series dating of corals (for high-sea stands back to about 500,000 years old) and strontium-isotope measurements of mollusks (for high-sea stands older than about 300,000 years). It is possible to determine the past water temperatures of the oceans during these high sea stands by detailed paleozoogeographic interpretations of fossil mollusk assemblages, a time-tested traditional method of paleoclimatic studies in marine settings. We also study the effects of sea level changes on coastal river systems and dunes.

The geomorphic record of sea level change on an uplifting coastline

Terrace ages in relation to normalized oxygen isotope composition — In this figure, the lower graph shows oxygen isotope variations in foraminifera in deep-sea cores (Imbrie and others, 1984), which are excellent records of glacial-interglacial cycles. Oxygen isotope periods are called "stages," with odd-numbered stages corresponding to warm interglacial periods, such as the present (oxygen isotope stage 1). Cold glacial periods are given even-numbered stage names, such as the last major ice age (oxygen isotope stage 2). Because sea level is low during glacial periods (ocean water is taken up to form ice sheets) and high during interglacial periods (ice sheets melt and return water to the ocean), the oxygen isotope curve shown here for the past ~400,000 years is to a great extent a record of sea level fluctuations as well as a record of temperature fluctuations.

On a tectonically active coast, such as California, changes in sea level are recorded as marine terraces, wave-cut benches that formed in the surf zone during interglacial periods, but are now above sea level due to uplift. Wave-cut benches also form during glacial periods, but these are offshore and underwater during interglacial periods. If uplift is continuous over time, a “stair step” type of landscape develops, as shown here, with each successively higher terrace corresponding to a successively older interglacial period.

[Reference: Imbrie, J., Hays, J.D., Martinson, D.G., McIntyre, A., Mix, A.C., Morley, J.J., Pisias, N.G., Prell, W.L., Shackleton, N.J., 1984, The orbital theory of Pleistocene climate: Support from a revised chronology of the marine δ¹⁸O record, in Berger, A., Imbrie, J., Hays, J., Kukla, G., and Saltzman, B., eds., Milankovitch and climate: Understanding the response to astronomical forcing: Dordrecht, D. Reidel Publishing Company, p. 269-305.]

The geomorphic record of sea level change on a stable or subsiding coastline

Normalized oxygen isotope composition in relation to a tectonically stable coast — Shown here is the same oxygen isotope curve as above, over the past ~400,000 years. On a tectonically stable coast, such as Florida, the land is not uplifting and because the coast is a lower-energy one, wave-cut benches are not as common as in California. However, coral reef growth can take place in favorable locations and the tops of some coral reefs are found just a bit below sea level at the time of growth. Thus, past interglacial high-stands of sea are recorded as coral reef limestones, stacked one on top of the other (see diagram). Deeper limestones are progressively older and each successive reef is marked by a buried soil (paleosol) that formed during the intervening glacial period, when sea level was low.

Why is this research important?

One of the most pressing issues in studies of climate change is the possible rise of sea level due to loss of major ice sheets, which would impact population, infrastructure, and habitats along the world's coastlines. It is not known which polar ice sheets (Greenland, West Antarctic, East Antarctic) are most at risk for mass loss that could contribute to sea level rise. Furthermore, it is not known what the possible magnitude of sea level rise is under interglacial climate conditions, how rapidly sea level may rise, or how long high sea levels may be retained. The goals of this project are to shed light on these questions by studying warm climate analogs of the geologic past.

World map showing location of major ice bodies and estimated sea level rise contributed by their melting — When past sea levels were higher, where did the water come from? Here are the possibilities, with the amount of sea level rise they could provide now.

How high was sea level during the last interglacial period?

Windley Key quarry with indication of sea level during the last interglacial period — Paleo-sea level = 5.5 meters + 3.0 meters = 8.5 meters above presentOn a tectonically stable coast such as the Florida Keys, we can estimate how high sea level was in the past, because fossil coral reefs give us a “high water mark” for a past interglacial period. Here at Windley Key, the top of a coral reef is 5.5 meters above modern sea level and has been dated to ~120,000 years ago (oxygen isotope stage 5), the last interglacial period. The types of corals in this reef require water depths of at least 3 meters. Thus, sea level ~120,000 years ago must have been 5.5 meters above present, plus at least 3 meters more (for habitat depth), indicating at least 8.5 meters above present.

Limestone in Miami with indication of sea level during the last interglacial period — Paleo-sea level = 7.5 meters (top of shoal) + 1.0 meter (depth for ooids) = 8.5 meters above presentAnother type of sea level record in Florida is from limestones composed of ooids, tiny, egg-shaped grains of sand that precipitate from sea water inorganically. Shallow areas where ooids form at present in the Bahamas are typically in water about 1 meter deep. Thus, here in downtown Miami, Florida, the Miami Limestone (composed of ooids) is found at an elevation of about 7.5 meters above sea level. It has also been dated to the last interglacial period, ~120,000 years ago (isotope stage 5). When this elevation is added to the typical depth of formation of ooids (1 meter of water), we infer a past sea level of about 8.5 meters above present, very similar to the Florida Keys coral reef record.

Key unanswered questions about past interglacial sea levels

The research conducted by this project attempts to answer these questions raised in the Intergovernmental Panel on Climate Change (IPCC) 2014 report:

The timing of past interglacial sea-level high stands
Marine paleotemperatures during past high-sea stands
Magnitudes of past interglacial high-sea stands
Rates of sea level rise

Results

What have we learned from the last interglacial?

What fossil corals can tell us

Corals are the only organisms that take up uranium from the ocean. Thus, they are the only fossils that we can use for high-precision uranium-series dating, one of our major tools to determine ages of marine deposits.

Florida Keys fossil reef corals, fossil corals from Puerto Rico, California fossil solitary corals, and fossil coral from Isla Guadalupe, Baja California

Charts showing the relation between isotope ratio and time — **We can date corals with *two* uranium-series clocks:** The ratio of 230Th/234U as well as the ratio of 234U/238U in a sample can tell us the age in years of the sample.

How long did the last interglacial period last?

These maps show the places where we have found either fossil coral reefs (tropical locations) or coral-bearing marine terraces (California) that have been dated by uranium-series methods to the last interglacial period. On each map, you can see the range of ages we have gotten from dating these corals that tell us how long this high-sea stand lasted.

Map of western Atlantic study locations showing Uranium-series ages — Uranium-series ages of reefs we have dated in the western Atlantic Ocean. Best estimates: A high sea level that lasted 8,000 to 14,000 years.

Map of Pacific study locations showing Uranium-series ages — Length of the last interglacial—Pacific. Uranium series ages on the west coast give us the best estimate as 123,000 to 114,000 years. A high sea level that lasted at least 9,000 years.

Map of Oahu, Hawaii study locations showing Uranium-series ages — Uranium-series ages of the last-interglacial Waimanalo reef on O'ahu, Hawai'i. A high sea level that lasted at least 12,000 years, and possibly 21,000 years.

How high was sea level in the last interglacial period?

Last-interglacial sea level as determined by dated corals — Elevations of corals we have dated from last-interglacial marine deposits on tectonically stable coastlines indicate a paleo-sea level of +5 meters to +10 meters.

National Parks that could be threatened by higher sea level — National Parks and Seashores on coasts that could be affected by future sea level rise.

Shoreline of Florida, USA as it looked during last interglacial period — What would Florida have looked like during the last interglacial period when sea level was several meters higher than present?: The blue line show the present coastline, but much of southern Florida would have been underwater, as well as sites where many of Florida's coastal cities are situated.

How warm was the ocean during the last interglacial period?

It is possible to determine the past water temperatures of the oceans during these high sea stands by detailed paleozoogeographic interpretations of fossil mollusk assemblages, a time-tested traditional method of paleoclimatic studies in marine settings. These maps show how marine invertebrates, now found only in warm waters farther south at present, expanded northward beyond their present ranges, during a warm last interglacial period.