Revised Postglacial Sea-Level Rise and Meltwater Pulses from Barbados

Lay Summary authored by Paul Blanchon. Read the full paper here:

The precise timing and rate of postglacial sea-level (SL) rise not only provides key insight into the nature of ice sheet disintegration during the last deglacial warming ~15 thousand years ago, but also helps constrain the response to future warming. One of the most complete reconstructions of postglacial SL rise is from Barbados where thick, back-stepping, sequences of the reef-crest coral Acropora palmata have been recovered in cores from the shelf and slope off the south coast. As seen in the figure below, successive reconstructions of SL using the Barbados data show rapid accelerations in SL rise at ~14 and 11 thousand years ago, and these have been linked to pulses of ice and meltwater discharge during the most rapid phase of ice-sheet decay. Similar meltwater pulses have subsequently been found in other SL reconstructions, but their timing, depth and magnitudes show significant differences with those from Barbados.

Successive postglacial sea-level reconstructions from reef-crest drill-core sequences at Barbados. The first 3 reconstructions assume that all dated corals are in growth position and so only plot the age and uplift-corrected depth of individual colonies. They consequently show variations in the timing and magnitude of the meltwater pulses as new coral data are added. Our revised reconstruction, however, shows that many of these data points are from coral clasts. In addition to accounting for this bias, we also add key stratigraphic data, such as the depth of tops and bases of reef-crest units, which allow for a more accurate assessment of the timing and magnitude of meltwater pulses. Figure provided by P. Blanchon.

To help resolve these differences, we re-examine the stratigraphy and growth history of the reef-crest sequences at Barbados and find that rather than consisting only of in-place coral colonies (as previously claimed), they are in-fact composed of a mixture of in-place colonies and large clasts generated by skeletal fragmentation and transport during hurricanes. This finding is important because the downslope transport of clasts complicates the assumption that the reef-crest corals grew within a few metres of SL, and so necessitates a reappraisal of the SL reconstruction and how the depth and magnitude of melt-water pulses are defined.

Classic vista of a coral-reef crest composed of dense thickets of Acropora palmata with wave-oriented branches. Photo credit: P. Blanchon.

By accounting for the bias created by downslope clast transport, we provide a revised SL reconstruction and show that the deepest A. palmata units were deposited on a slope next to deep-water corals, not on a shallow elevated reef-crest structure as previously thought. As a consequence, the onset of the first meltwater pulse, MWP-1a, cannot be identified from Barbados, which explains it’s earlier timing in other SL reconstructions. We also show that the depth and magnitude of the second meltwater pulse, MWP-1b, is best defined from the difference in elevation between the top of the reef-crest that was drowned by the pulse, and the base of the reef-crest that back-stepped after the pulse ended. Using these elevations, with adjustments for water depth, we estimate that the second meltwater pulse was 5 m smaller, shallower, and occurred 150 years later than previously claimed. This smaller magnitude may explain why MWP-1b has not been recognized in the other major SL reconstruction at Tahiti: it may not have been large enough to shift the reef crest into waters with deeper corals, thereby allowing it to simply regrow and catch-up with sea level. So although these new findings help reconcile differences with other reconstructions, they also highlight the limitations and uncertainties in reconstructing SL from more complex reef sequences.

The aftermath of a hurricane, with the decimated reef crest being reduced to a field of rubble. Looking at the contrast to the picture above, it’s easy to understand how our fairweather perspective and limited lifespans bias the interpretation of fossil reef-crest sequences, like those found at Barbados. Photo credit: P. Blanchon.

Full paper: Blanchon, P., Medina-Valmaseda, A. and Hibbert, F.D., 2021. Revised Postglacial Sea-Level Rise and Meltwater Pulses from Barbados. Open Quaternary, 7(1), p.1. DOI:

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A review of fossil burial laws and preservation studies in Southeast Asian caves

Lay summary authored by Holly Smith. Read the full paper here:

Caves are critical sites for understanding Quaternary fossil remains around the world. A large percentage of fossil sites from this time period are from caves. It is therefore critical to study how burial, decay and preservation processes impact the cave fossil record. These studies are important to understand how well caves preserve the signals of past diversity and ecology. Southeast Asia has a rich and interesting fossil history, however, the mechanisms of long-term fossil preservation in this region are poorly understood. This is in part due to dating complexities and poor preservation potential in tropical environments. The literature on Southeast Asian caves is summarised in order to critically analyse the scientific methods used over time. We address the issues inherent in the conventional methods used in Southeast Asian cave studies due to the associated difficulties of discerning the mechanisms of cave formation, depositional history and fossil accumulation.

Dr Gerrell Drawhorn of California State University, USA and Dr Yan Rizal of Institut Teknologi Bandung, Indonesia excavate Pleistocene fossil remains from a breccia deposit in Lida Ajer cave, Sumatra. Credit: Holly Smith, with permissions.

It is considered that calcareous cave deposits called breccia that accumulate in limestone caves may hold the key to resolving this issue. Breccia are the primary source of data in the literature as they occur frequently and are often fossiliferous. A key knowledge gap lies in the absence of contextual data stored in the sediment of these deposits. The internal structure of breccia is typically disregarded or destroyed during excavation. It is considered that there are significant opportunities for future research in the use of digital techniques to three-dimensionally reconstruct the internal material properties of fossiliferous breccia.

PhD candidate Holly Smith of Griffith University, Australia excavates a breccia block sample from Lida Ajer cave, Sumatra. Credit: Holly Smith, with permissions.

There is excellent breccia potential for establishing the dominant factors responsible for accumulation and preservation of cave vertebrate remains. Our analysis highlights that breccia deposits have a great deal of evidence in store for anthropological and palaeontological studies in Southeast Asian caves in the future.

Full paper: Smith, H.E., Morley, M.W. and Louys, J., 2020. Taphonomic Analyses of Cave Breccia in Southeast Asia: A Review and Future Directions. Open Quaternary, 6(1), p.13. DOI:

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A Review of Ethnographic use of Wooden Spears and Implications for Pleistocene hominin hunting

Lay summary authored by Annemieke Milks (Institute of Archaeology, UCL, London, GB). Read the full paper here:

In the year 1911 Samuel Hazzledine Warren, an amateur British geologist, made an unprecedented discovery – a wooden artefact from Pleistocene sediments in Clacton-on-Sea (UK) – which he reported to the Royal Geological Society as being ‘the point of a palaeolithic spear’. In the same year, the German geologist and mining engineer Fritz Noetling described the design and uses of Aboriginal Tasmanian wooden spears for Papers and Proceedings of the Royal Society of Tasmania. Noetling described these one-piece spears crafted from a single piece of wood as ‘simple’ and ‘primitive’, and characterised those who made them using racist language. Unfortunately, Noetling cannot be singled out in his use of such language or analogical reasoning: several archaeologists and palaeoanthropologists subsequently made similar arguments that connected people using simple technologies with cognitive deficiencies. Furthermore, academics have often referenced wooden spears used in small-scale societies in selective and arguably damaging ways. This relates both to the interpretations of their technologies, and by extension, the frameworks that these observations provided for evaluating human evolutionary trajectories.

Figure 1: One of the few remaining Aboriginal Tasmanian spears (M.2723), located in the Tasmanian Museum and Art Gallery. These spears, made of woolly teatree (Leptospermum lanigerum),were thrown by hand at great distances, and are exceptionally long and thin. Photo credit: A. Milks, with thanks to the Aboriginal Advisory Council and Tasmanian Museum and Art Gallery for permission to study and photograph these objects.   

Interpretations like that of Fritz Noetling, which linked one-piece wooden tools with ‘primitivism’, have unfortunately been repeated by researchers who interpreted Pleistocene wooden spears as simple and ineffective, particularly in relation to subsequent innovations by Homo sapiens. This characterisation, partly based on their being one-piece tools, ignores key design features. For example, carefully placed points of balance and tapering allow spears to function as projectile weapons, while specific shaping of the wood facilitates both durability and penetration into prey. Continuing the analogy, researchers proposed that technological and performance limitations resulted in severe constraints on how and what hominins wielding such weapons could hunt. However, as is shown in this review paper, these analogies are based on incomplete and selective ethnographic data, or unskilled experiential use of replicas. They do not harmonise with the mounting evidence that Pleistocene hominins inhabited many different environments and terrains, had sophisticated and variable technologies, and were hunting many different types of prey.

Figure 2: A selection of the animals reported to be hunted with wooden spears in the ethnographic literature. Clockwise from top left: Jaguar (Panthera onca), beaver (Castor canadensis), tapir (Tapirus terrestris), and emu (Dromaius novaehollandiae). Image created by Annemieke Milks from the following open sources: Jaguar: By Charles J Sharp – Own work, from Sharp Photography,, CC BY-SA 4.0, Beaver: By Daderot – Own work, CC0, Tapir: By Charles J Sharp – Own work, from Sharp Photography, sharpphotography, CC BY-SA 4.0, Emu: By John Robert McPherson – Own work, CC BY-SA 4.0,

This paper presents the first systematic review of the use of wooden spears by recent small-scale societies. It includes examples of the use of these weapons for hunting and violence, and documents how and where they were used. Collectively, it shows that wooden spears were used in diverse climates and ecological settings, including both open and forested environments. People used them to hunt a wide variety of both dangerous and docile prey including rodents, large herbivores, carnivores, flightless birds and aquatic mammals. Hunting strategies were similarly varied, and wooden spears are clearly adaptable weapons that can be used in creative and multifunctional ways. A reinterpretation of Pleistocene hunting based on these ethnographic data and further supported by recent experiments should help resolve some current tensions in the discipline. Indirectly, it supports recent archaeological evidence that Pleistocene hominins were more technologically and behaviourally flexible than they are often suggested to have been.

Full paper: Milks, A., 2020. A Review of Ethnographic Use of Wooden Spears and Implications for Pleistocene Hominin Hunting. Open Quaternary, 6(1), p.12. DOI:

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People, Shellfish, and Estuaries in Ancient Island and Mainland Southern California

Lay summary authored by Torben Rick (Department of Anthropology, National Museum of Natural History, Smithsonian Institution, Washington, DC, US). Read the full paper here:

Archaeologists have long been interested in understanding the relationships between past environmental change and human diet and settlement choices. This is particularly true for coastal regions, including estuaries where fresh water and marine systems merge to create dynamic habitats. Glacial to interglacial sea level rise, particularly between about 15,000 and 5000 years ago, created a number of estuaries around the world. These estuaries provided people with rich sources of food, especially shellfish like oysters, clams, and other mollusks.

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Figure 1. Location of estuary sites in the Santa Barbara Channel Region dated between about 9000 and 5000 years ago, including the Abalone Rocks Estuary on Santa Rosa Island. Figure by Leslie Reeder-Myers and Torben Rick.

This study explores the importance of estuaries versus rocky shore marine habitats for human diet on the islands and mainland coast of California’s Santa Barbara Channel (Figure 1). Previous research demonstrated that estuaries were important for people living in the Santa Barbara Channel from >11,000 years ago through about 5000 years ago, and in some places through the 19th century. While ancient estuaries appear to have been relatively common along the Santa Barbara mainland coast, they were rare to absent on the adjacent northern Channel Islands, with just one well documented island estuary (Abalone Rocks) (Figure 2). Despite knowledge about this ancient island estuary, we still knew little about the productivity of this estuary and its importance for human subsistence and settlement compared to the more extensive mainland estuaries.

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Figure 2. Eastern Santa Rosa Island looking at the Pacific Ocean and the Abalone Rocks Marsh, the remnant of the former Abalone Rocks Estuary (center right of the photo). Photo by T. Rick.

Excavation and subsequent analysis of archaeological shellfish remains from island archaeological sites near Abalone Rocks and similarly aged sites near mainland estuaries provide the first detailed means to compare the importance of mainland versus island estuaries in southern California. This study demonstrates that estuarine shellfish were considerably more abundant at most mainland sites, with shellfish from the Abalone Rocks Paleoestuary largely supplementary to those from rocky shore habitats on the Channel Islands. Estuarine shellfish largely disappear from island sites by about 5000 years ago, effectively ending human use of estuaries for island subsistence. While focused on California’s Santa Barbara Channel, this study illustrates the importance of archaeology for helping understand the relationships between past human diet, land use, and environmental change.

Full paper: Rick, T.C., 2020. Early to Middle Holocene Estuarine Shellfish Collecting on the Islands and Mainland Coast of the Santa Barbara Channel, California, USA. Open Quaternary, 6(1), p.9. DOI:

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Reconstructing a giant fossil dormouse

Lay summary authored by Jesse Hennekam. Read the full paper here:

The ‘island effect’ is an evolutionary phenomenon that describes how animals often evolve substantial changes in body size when isolated on islands. More specifically, large mammals become smaller (e.g. dwarfed elephants and hippopotamuses), whilst small mammals (notably rodents, lagomorphs and insectivores) become larger relative to their mainland relatives. This latter process of insular gigantism is well known from Mediterranean islands over the last 12 million years. In particular, dormice have experienced this phenomenon on multiple occasions, with the exceptionally large species, Leithia melitensis from the Pleistocene of Sicily and Malta, reaching the size of a domestic cat. Although many fossils of the lower jaw are known of this species, the skull was previously only known from fragmentary material.

image 1- skull

Figure 1: A sketch of the lateral view of the composite skull of the gigantic dormouse Leithia melitensis. Credit: James W. F. Sadler.

In their study published in Open Quaternary, the lead author noticed a section of cave floor on display at the Palermo Museum. This floor segment had been excavated from Poggio Schinaldo Cave, Sicily, during the construction of a motorway in the 1970s. A number of rodent skulls could be seen on the surface of this rock segment. By investigating the dentition of one of the skulls, it was clear that these fossils were in fact the giant dormouse Leithia melitensis. Considering the rarity of cranial material belonging to this species, the segment was transported to Basel, Switzerland. The microCT scanning facility here was used to scan two sections of the cave floor, enabling the researchers to visualise the interior of the rock. The rock segment turned out to be a conglomerate of fossil material including at least five partial skulls of Leithia melitensis. Virtual reconstruction based on the differences in density between bone and matrix was used to extract 3D models of the partial skulls. The best preserved features of these skulls were then combined to create an almost complete composite skull. This composite represents the best approximation of the complete skull morphology of Leithia melitensis known so far, and will aid palaeontologists in making inferences about the lifestyle and behaviour of this fascinating extinct giant dormouse.

image 2- reconstruction

Picture 2: Reconstruction of the Pleistocene giant dormouse, Leithia melitensis (left), and its nearest living relative the garden dormouse, Eliomys quercinus (right). Credit: James W. F. Sadler.

Full paper: Hennekam, J.J., Herridge, V.L., Costeur, L., Di Patti, C. and Cox, P.G., 2020. Virtual Cranial Reconstruction of the Endemic Gigantic Dormouse Leithia melitensis (Rodentia, Gliridae) from Poggio Schinaldo, Sicily. Open Quaternary, 6(1), p.7. DOI:

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Investigating the use of microfossils to reconstruct past sea levels from salt-marsh environments in northern Georgia, USA

Lay summary authored by Huixian Chen. Read the full paper here:

Changes in sea level vary geographically and with time and in response to a variety of processes. These include the melting of land-based ice from ice sheets (Antarctica and Greenland) and glaciers, thermal expansion of the oceans, vertical motions of land (e.g. earthquakes) and by local processes such as the compaction of sediments. Understanding past sea levels provides important background information that is useful for determining how sea levels may change in the future. Our understanding of past sea levels stems from a variety of geological reconstructions that utilize evidence from natural environments (e.g. salt marshes, mangroves and corals). Along the Atlantic coast of North America, salt marshes have provided abundant information of how and why sea levels have changed during the past ~10000 years.


Salt-marsh environment at Georgetown, northern Georgia, U.S. Atlantic coast. Credit: H. Chen.

The use of salt marshes in providing such information stems from their relationship with tidal levels. Salt-marsh environments typically exist between mean tidal level up to the highest astronomical tide. Further to this, microfossils such as foramnifera living on the surface of the salt marsh may be used to further subdivide the environment into narrower vertical ranges. This is due to different foraminiferal species preference to living conditions including level of salinity and duration of exposure as the tides rise and fall. If we understand and document where these different species live today, we can apply this information to fossil counterparts in sediment cores of known age through methods such as radiocarbon dating and reconstruct how sea levels have changed with greater accuracy.


Scanning electron microscope images of the most abundant foraminiferal species observed in the salt marshes at Thunderbolt and Georgetown. From left to right: Haplophragmoides wilberti, Miliammina fusca, Ammoastuta inepta. Haplophragmoides wilberti is typically found at higher elevations in the salt marshes. Credit: H. Chen.

In this study, we investigated the surface (top 1cm) distributions of foraminifera from two salt marsh sites, Thunderbolt and Georgetown, in northern Georgia to document where different species live in relation to tidal levels. To do this, we set up multiple surface transects and collected surface sediment for analysis under a microscope. We show that certain species are more prevalent in different areas of the salt-marsh environment and thus have different relationships with tidal levels. We also sampled short (50 cm) sediment cores and analyzed the depth to which living (at the time of collection) foraminifera penetrate down into the subsurface to assess their potential implications for when reconstructing sea level from these environments.

Full paper: Chen, H., Shaw, T.A., Wang, J., Engelhart, S., Nikitina, D., Pilarczyk, J.E., Walker, J., García-Artola, A. and Horton, B.P., 2020. Salt-Marsh Foraminiferal Distributions from Mainland Northern Georgia, USA: An Assessment of Their Viability for Sea-Level Studies. Open Quaternary, 6(1), p.6. DOI:

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Chronostratigraphy, Site Formation, and Palaeoenvironmental Context of Late Pleistocene and Holocene Occupations at Grassridge Rock Shelter (Eastern Cape, South Africa)

Lay summary authored by Christopher Ames. Read the full paper here:

The Grassridge archaeological site is a sandstone rock shelter located in the grassland foothills of the Stormberg Mountains in the Eastern Cape of South Africa. Dr Hermanus Opperman of Fort Hare University first excavated the site in 1979. He and his team identified a 1.5 m deep sequence of human occupation at Grassridge, which they divided into two major episodes: a 50–70 cm thick upper portion that is 7,000-6,000 years old, and an underlying 50–80 cm thick portion dated to 36,000 years old at the bottom. This indicates that peopled lived at Grassridge in the lead up to the Last Glacial Maximum (29,000-14,000 years ago), but that it was potentially uninhabited at the height of cold and dry temperatures of the last glacial itself. Located approximately 200 km inland from the Indian Ocean coastline, Grassridge sits at an important environmental intersection between the Drakensberg Mountains to the northeast, the South African coastal zone to the south and southeast, and the interior arid lands of southern Africa to the northwest; the grasslands surrounding Grassridge would thus have been sensitive to past climate change.


The modern grassland and neighbouring hills that surround Grassridge rock shelter. Photo credit: Lisa Rogers, 2015.

Building on this knowledge, the Grassridge Archaeological and Palaeoenvironmental Project (GAPP) initiated new work at Grassridge in 2014 to reanalyse the stratigraphic sequence and initiate a new excavation programme. In this study, we present the results of our stratigraphic reanalysis, which combines luminescence and radiocarbon dating techniques with sedimentological characterisation and analysis of preserved microbotanical remains to establish the nature of archaeological site formation as well as the palaeoenvironmental contexts of the human occupations. Our work refines the previous excavation results, indicating a history of pulsed human occupation at Grassridge from 43,000-28,000, 13,500-11,600, and 7,300-6,800 years ago. A cool, dry grassland environment is associated with the 43,000-28,000 year old occupation, which transitions to a warmer and wetter grassland environment with more trees—similar to modern conditions—by 7,000 years ago. Moreover, the record at Grassridge confirms the continuation of Middle Stone Age technology at sites in the interior of South Africa until 30,000 years ago or younger, and places Grassridge as one of very few interior sites that preserve occupation from the Younger Dryas (13,000-11,500 years ago). Such a pulsed occupational sequence is characteristic of the regional archaeological record in the high elevation grasslands for the period before, during, and after the Last Glacial Maximum (45,000-5,000 years ago).


Photo 2: Christopher Ames sampling the lowermost layers of the Grassridge stratigraphic sequence. Photo credit: Benjamin R. Collins,

This pattern of human occupation seems linked to short distance, rapid environmental changes along an altitudinal gradient in response to rapidly fluctuating palaeoclimatic conditions that occurred across the last glacial to interglacial transition. Only continued joint archaeological and palaeoenvironmental research in the high elevation grasslands and surrounding regions will help evaluate this hypothesis.

Full paper: Ames, CJH, Gliganic, L, Cordova, CE, Boyd, K, Jones, BG, Maher, L and Collins, BR. 2020. Chronostratigraphy, Site Formation, and Palaeoenvironmental Context of Late Pleistocene and Holocene Occupations at Grassridge Rock Shelter (Eastern Cape, South Africa). Open Quaternary, 6: 5, pp. 1–19. DOI:

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DNA from underwater archaeological sites

Lay summary authored by Lisa Briggs. Read the full paper here:

The methods used to study ancient DNA have progressed rapidly over the last few decades. As more submerged archaeological sites are discovered and explored, it is possible to apply these new methods and technological advancements in DNA research to recently discovered archaeological material from maritime and underwater sites. Environmental DNA, in particular, has been shown to be particularly important in characterising the human activities that might have taken place at ancient sites, both on land and under the sea. Given these developments, and the heightened awareness of how important it is to characterise the environmental samples taken from archaeological sites, it is necessary to reconsider previously published studies conducted on archaeological material from submerged sites.


Underwater archaeologist Lisa Briggs recovering artefacts from the Queen Anne’s Revenge shipwreck (Photo credit: W. Welsh)

This paper explores the pitfalls, promise and future directions of ancient DNA research on archaeological material recovered from submerged sites. First, a selection of previously published studies is assessed in light of the new methods and technological advancements that have now been made available to archaeological scientists. Past research conducted on human and animal bones, waterlogged plant remains, and archaeological ceramics is considered. Finding ways to authenticate the DNA recovered as ancient is of paramount importance. Next, the promise that DNA analysis holds for artefacts and environmental samples recovered from underwater sites is explored. Submerged prehistoric settlement sites, ancient shipwrecks, and harbour sites hold particular promise for the application of new methods designed to maximise the recovery of ancient DNA. Finally, the future directions of ancient DNA research on archaeological material from underwater sites are considered. Understanding what DNA is naturally present in the seafloor sediment that accumulates over archaeological sites, what DNA is naturally present in seawater, and how DNA breaks down in the marine environment will greatly enhance our ability to assign an archaeological meaning to DNA discovered on ancient underwater sites.

Full paper: Briggs, L., 2020. Ancient DNA Research in Maritime and Underwater Archaeology: Pitfalls, Promise, and Future Directions. Open Quaternary, 6(1), p.3. DOI:

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Identifying the Greatest Earthquakes of the Past 2000 Years at the Nehalem River Estuary, Northern Oregon Coast, USA

Lay summary authored by Alan Nelson. Read the full paper here:

Will the next great earthquake at the Cascadia subduction zone, whose fault dips beneath the coasts of British Columbia, Washington, Oregon, and northern California, be a giant (magnitude 9) like the earthquake and tsunami that killed thousands in Japan in 2011, or the similar Cascadia earthquake in AD 1700? How often do such great earthquakes and their accompanying high tsunamis strike this region? To answer these questions geologists study past great earthquakes beneath coastal wetlands where evidence of them is best preserved.

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Part of the field crew describing earthquake stratigraphy in cores, and taking their GPS locations, in Botts marsh, near the mouth of the Nehalem River, northern Oregon, in July 2009 (Photo: A. Nelson).

Longer fault ruptures produce larger earthquakes, so much effort has gone into determining the length of the Cascadia fault that ruptured during each great earthquake of the past few thousands of years. Usually this has been attempted by radiocarbon dating coastal wetland sediment that records sudden subsidence during earthquakes or their accompanying tsunamis at a series of sites along the coast. But the errors on radiocarbon dating are still too large to distinguish between one giant earthquake rupturing 500-1200 km along the coast, or a series of merely great earthquakes over a few years to a few decades progressively rupturing shorter sections of coast.

At the Nehalem River estuary of northern Oregon we used a more direct method of estimating the magnitude of prehistoric subduction-zone earthquakes—by reconstructing the amount that the coast suddenly dropped down during each great earthquake. The most precise way to measure this sudden subsidence is to study changes in fossil microscopic tidal plants (diatoms) and animals (foraminifera) in tidal sediment at each wetland over thousands of years. Beneath the Nehalem wetlands we identified changes in sediment that suggested three times when the coast suddenly dropped down. Using new statistical methods, combined with information recently gathered by others about how high above low tide modern diatoms and foraminifera typically live, we measured 0.6-1.7 m, 0.3-1.5 m, and 0.6-1.5 m of subsidence during the three earthquakes. Study of fossil diatoms independently confirms similar amounts of subsidence during the two younger earthquakes. Our radiocarbon ages show that the earthquakes occurred about 320, 900, and 1500 years ago.

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Rob Witter (USGS) measures the depths of the upper stratigraphic contacts of two subsided marsh soils, contact A (at 1.1 m on the measuring tape, below his left hand) and contact B (at 0.18 m on the measuring tape, between his knee and ankle), at an outcrop near the mouth of the Nehalem River, northern Oregon, in June 2009. Contact A formed through sudden subsidence of the marsh during the great Cascadia earthquake of 1700 CE. Contact B formed during an earlier great earthquake about 1200-1000 years ago (Photo: A. Nelson).

Although no one can predict the magnitude or time of a future great earthquake, by learning more about past subduction-zone earthquakes, and how their fault ruptures may have varied along the coast, seismologists and geologists hope to forecast the probability and magnitude of a future great earthquake at Cascadia.

Full paper: Nelson, A.R., Hawkes, A.D., Sawai, Y., Engelhart, S.E., Witter, R., Grant-Walter, W.C., Bradley, L.-A., Dura, T., Cahill, N. and Horton, B., 2020. Identifying the Greatest Earthquakes of the Past 2000 Years at the Nehalem River Estuary, Northern Oregon Coast, USA. Open Quaternary, 6(1), p.2. DOI:

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Rock-boring organisms tell us about sea-level fluctuations in a warmer world

Lay summary authored by Michaela Falkenroth. Find the full paper here:

The Last Interglacial, 130.000 until 115.000 years ago, was the last time global mean temperatures and global sea-level were higher than today. This has put the period onto the agenda of scientists, who are asked to provide communities with scenarios of coastal evolution in a warming world. Researchers currently conduct a global database of areas, where remains of last interglacial shorelines still present today.

One of these areas is the coastline of Oman, that borders the western Indian Ocean. While the Sultanate`s coastlines are for the most part preserved in their natural state without any anthropogenic influence, a true rarity among the coastlines of the world, their sea-level history is still understudied.

Last Interglacial Paleoshoreline with notch (left) and beachrock (right) in Sur

Last Interglacial Paleoshoreline with notch (left) and beachrock (right) in Sur (Photo: M. Falkenroth).

With this work we wish to lay the ground for research on last interglacial shorelines in Oman. Our example is a very well preserved tidal notch at Sur Lagoon that is located 3.93 m above recent mean sea level. Tidal notches are carved into rocky shorelines by wave action and organisms that bore into hard substrates. These organisms can be clams, sponges or worms that create cavities in a rock-surface by chemical dissolution and mechanical action. The cavities function as shelter for the animal or are a side effect of its feeding behaviour.

A tidal notch is an indicator for sea-level as its deepest point lies at mean sea-level while its floor and roof represent the tidal range. Because the process of notch-formation is rather slow, they form in periods during which sea-level remained stable. This also means that the notch itself will not significantly change if mean sea-level rises or falls for a short period of time.

Rocksurface with Lithiphaga borings

Rocksurface with Lithiphaga borings (Photo: M. Falkenroth).

The organisms living within the notch however, will change their behaviour almost instantaneously, as their habitat is restricted to a certain bathymetry. The common boring clam Lithophaga needs to be permanently submerged. When sea-level falls, Lithophaga will abandon all newly exposed surfaces and if sea-level rises the clams will penetrate the newly submerged rocksurface within a couple of years.

This can lead to a mismatch between the shape of a tidal notch and the distribution of organisms within it, as they act on different timescales. The tidal notch in Sur shows Lithophaga borings above the notch roof, which are interpreted as remains of a short term sea-level rise subsequent to notch-formation and tell a story of last interglacial sea-level variability.

Full paper: Falkenroth, M., Adolphs, S., Cahnbley, M., Bagci, H., Kázmér, M., Mechernich, S. and Hoffmann, G., 2020. Biological Indicators Reveal Small-Scale Sea-Level Variability During MIS 5e (Sur, Sultanate of Oman). Open Quaternary, 6(1), p.1. DOI:

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