Paleoshorelines: Time capsules of the ocean’s ancient shorelines

Written in the coasts

To appreciate the oceans today, research sometimes must chase the past, to discover how iconic features such as coral reefs, dunes, and sand bars have been etched into the history of the coasts. Marine archaeology is one branch of marine and oceanic science that seeks to improve our understanding of how these iconic structures formed and remained preserved through geologic time, which encompasses millions of years. Recent advances in marine science, such as side-scan sonar and bathymetric surveys, have allowed for marine archaeologists to unravel much more than gargantuan shipwrecks.

When you go to the beach or pick up a postcard with breathtaking coastal landscapes, you are witness to a timeline of ancient erosional and depositional oceanic processes. An emerging research interest within marine archaeology focuses on oceanic histories written in sand, minerals, and skeletons of organisms that lived thousands, even millions, of years ago. These timelines are paleoshorelines, the remains of coastal landforms, such as beaches, sand dunes, and coral reefs. The survival of these ancient shorelines for thousands of years provides us with clues about past sea levels that fostered the preservation of these features.

Why do paleoshorelines matter?

Over the past 200 million years, the oceans have experienced large fluctuations in sea levels. The major drivers of sea level changes on long timescales include plate tectonics and changes in the volume of water contained in ocean basins and glaciers. Paleoshorelines formed during the Late Quaternary, a period on Earth that occurred between 500,000 and 1 million years ago.

Earth’s geologic time scale, a timeline of major importance for archaeologists and marine scientists. Photo credit: Wiki Commons.

Sea levels during this period were much lower than present, and a suite of environmental conditions allowed for the preservation of paleoshorelines.

Depending on how well these structures are preserved, paleoshorelines can capture a valuable record of environmental change and  inform our understanding of modern shelf ecosystems. Not only can these structures indicate the distributions of seabed features that provide important habitat for a variety of marine life, they also have potential to shed light on the location of coastal resources used by humans, and so are of archaeological significance.

Where can you find paleoshorelines?

Even though they sound so distant, paleoshorelines exist along the Florida coast and several Pacific islands, including Tonga.

Recently, researchers in Australia analyzed sea-level data collected from the Red Sea and the Australian continental shelf to determine the sea levels where ancient shoreline features developed, as well as provide some evidence for the biodiversity patterns seen around the coasts of the world. These researchers found that much of the shoreline features formed when sea-levels were 30-40 m lower than present.

Map of a hypothesized distribution of paleoshorelines. The highlighted portions of continents represent land above sea level, so where the highlighted edges meet the sea indicate the potential locations of paleoshorelines. Photo credit: Wiki Commons.

How do they form?

Two primary factors determine how well a paleoshoreline feature will be preserved: sediment type and time of formation. In general, recently formed structures should be preserved better because they have been subjected to shorter periods of erosion. Sediment type, sediment inputs, current, and wave regimes all interact to influence the development and preservation of relict coasts. The number of sea-level fluctuations that a coastline feature has experienced also influences the degree of preservation.

What about the role of  marine animals that make calcium carbonate shells, such as coral, algae, clams and mussels, in the formation and distribution of paleoshoreline structures? These animals are especially important in regions receiving low inputs of land-derived sediments. Because shell-building animals form reefs and large coastal barriers out of skeletal fragments, which can be rapidly cemented together, these structures remain highly preserved even in the face of fluctuating sea-levels. Thus, more sturdy structures such as coral reefs and cliffs, have a higher potential to be preserved over several episodes of sea-level fluctuations.

Featured image. Acoustic image of a relict barrier reef off the coast of Australia. Photo credit: Brooke et al. 2017 doi: Used with permission via Creative Commons.

In contrast to robust coral reefs and barriers surrounding the Great Barrier Reef, the silica-dominated eastern shelf offshore of Australia’s coastline south of the Great Barrier Reef hosts low-relief remnants and buried deposits of Late Quaternary structures. These shoreline features, due to their lack of strong calcium carbonate, are not as pronounced as the coral reefs, which provides strong evidence for the importance of sediment type in preservation of these structures.

Why paleoshorelines matter today

Paleoshorelines, even though they formed hundreds of thousands of years ago and have withstood the unrelenting tests of tides and waves, provide distinct zones of seabed complexity. This is especially true of coral reefs, with their three-dimensional structures, which create habitats and refuges for juvenile and adult animals. The community of algae and benthic (bottom-dwelling) animals such as clams, sea sponges, and fish living on complex paleoshoreline features can be quite different, and often more diverse, than adjacent communities living on flat, unstable seabed. For example, on the northern Australian shelf, calcium carbonate banks that formed as near-surface coral reefs during periods of lower sea level, in combination with other environmental variables, support biodiversity “hot spots.”

The presence of paleoshorelines may help explain global patterns of seabed diversity that other variables, such as salinity, temperature, and oxygen concentration, do not. For example, extensive populations of demersal (bottom-feeding) fish on the Australian continental shelf occur in depth zones of 70 to 100 m and 120 to 150 m. These depth zones cooccur with large-scale paleoshoreline features of drowned coral reefs and shoreline cliffs. These fish may occur in these depths because these structures foster abundant food, shelter, and abundant benthic biodiversity. Management and conservation of our coasts will have to take into consideration the biodiversity value of these ancient structures.

Linking past, present, and future

When we look to the horizon, above the crashing waves, we may wonder, what does the future of our oceans hold? What does a 500,000-year-old reef mean to us now, or in the future? The study of paleoshorelines provides a sense of stability in a world of rapid environmental change. Scientists have long expressed concerns about rising sea levels and increasing ocean acidification. Perhaps we can look to the past, to the persevering coral reefs, and realize that although the world is changing, our present Great Barrier Reef and reefs around the globe, will record the unfolding history, and hopefully we can work to keep these modern structures alive so that global biodiversity will thrive.

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