Friday, November 11, 2011

Did a tsunami destroy ancient Lechaion?

Did a tsunami destroy ancient Lechaion?

Did a tsunami destroy ancient Lechaion?

Posted on November 10, 2011 by dpettegrew In early July, Andreas Vött and his colleagues announced that sometime in the 6th century AD, a tsunami destroyed ancient Olympia, the famous site of pan-Hellenic athletic contests. In considering recent scholarship on historical tsunamis in the Gulf of Corinth, I pondered here at Corinthianmatters whether there was any evidence for tsunamis in the Corinthia. As it turns out, a workshop on tectonics and earthquake geology occurred in late September that included presentations on the subject. The papers at the workshop have now been published as short articles in Earthquake Geology and Archaeology: Science, Society and Critical Facilities and include two relevant articles, both of which consider tsunamigenic deposits at Lechaion:

Hadler, Hanna, Andreas Vött, Benjamin Koster, Margret Mathes-Schmidt, Torsten Mattern, Konstantin Ntageretzis, Klaus Reicherter, Dimitris Sakellariou, Timo Willershäuser, “Lechaion, the Ancient Harbour of Corinth (Peloponnese, Greece) destroyed by Tsunamigenic Impact,” pp. 70-73.

Koster, Benjamin, Klaus Reicherter, Andreas Vött, Christoph Grützner, “The Evidence of Tsunamigenic Deposits in the Gulf of Corinth (Greece) with Geophysical Methods for Spatial Distribution,” pp. 107-110.

Some of the same researchers (Hanna Hadler, Andreas Vött, Konstantin Ntageretzis, Timo Willershäuser) responsible for this work also published the Olympia tsunami. Indeed, the Olympia and Lechaion studies are both part of a broader study of tsunamis in the Ionian Sea and the Peloponnese. The Hadler et al. paper is especially intriguing. The abstract:

Lechaion, the harbour of ancient Corinth, is situated at the south-eastern extension of the Gulf of Corinth (Peloponnese, Greece). Due to extensive fault systems dominating the gulf, seismic activity is frequent and often related to landslides or submarine mass movements. Thus, the study area is highly exposed to tsunami hazard. By means of geo-scientific studies comprising geomorphological, sedimentological and geophysical methods, evidence of multiple palaeotsunami impact was encountered at the Lechaion harbour site and the surrounding coastal area. The detected tsunami signatures include allochthonous marine sediments intersecting quiescent harbour deposits, extensive units of tsunamigenic beachrock and geoarchaeological destruction layers. Our results suggest that the harbour at Lechaion was finally destroyed in the 6th century AD by strong tsunami impact I asked Dr. Richard Rothaus of Trefoil Cultural and Environmental Heritage if he would review the Lechaion piece as guest blogger. Rothaus offers an expert opinion in multiple ways. He published the seminal English article in OJA on the harbor of Lechaion, discussed the late antique phases of Lechaion harbor and basilica in his book Corinth: The First City of Greece (2000) and, in his role as coastal archaeologist in the Eastern Korinthia Archaeological Survey, took vibracores at Lechaion.

What follows is Rothaus’ review as well as a report on his own work at Lechaion. I have added several images as visuals for the discussion.

*************************** The Second International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering 19th-24th September 2011 contains several articles about the palaeoseismicity of the Corinthia, as Dr. Pettegrew has already indicated. I am particularly interested in the papers on Lechaion. I became interested in the harbor at Lechaion at the first INQUA meeting in Corinth, and worked with several palaeoseismologists, under the lead of Stathis Stiros in investigating the coseismic uplift on that harbor. Stiros and his team published a paper in Geoarchaeology suggesting a possible c. 600 BC construction date for Lechaion. The borings of marine mollusks Lithphaga lithophaga L. with a clear upper terminus were observed on the interior face of the limestone blocks lining the channel that connects the inner harbor to the Corinthian Gulf.

Lithophaga generally live in the sublittoral zone, and those that are established in the midlittoral zone are not long preserved due to bioerosion. The preservation of lithophaga borings on the wall has been interpreted as an indication of sudden coseismic uplift. Samples were removed from two shells in living position, and AMS radiocarbon dates indicate a date between the fifth and third century BC. The researchers have interpreted this to indicate a construction date prior to an uplift event of greater than 1 m (and that excludes correction for sea-level change). I was a coauthor on this paper, and also published an archaeological overview of the harbor at Lechaion that included that date in the Oxford Journal of Archaeology.

Lechaion Harbor mounds and moles viewed from Korakou to the east More importantly (at least for me) I also learned at that INQUA meeting that earthquakes and coseismic phenomena (e.g. uplift, tsunamis) are far more complicated than I ever suspected.

Subsequent to the meeting and those publications, I began a closer study of seismic and coseismic evidence in the archaeological record with my colleagues Eduard Reinhardt and Jay Noller. That study has taken many interesting turns, including visits within a few weeks of events to earthquake-stricken areas in Turkey in 1999-2000, and India in 2001. Those visits solidified our concerns that the evidence for earthquakes in the archaeological record is very, very difficult to read. Likewise we became increasingly aware of how localized co-seismic phemomena can be.

Some of our results from Turkey were published in Natural Hazards, and I used the results from India to highlight the unreliability of the literary record here. Most recently I assisted Simon Donato in identifying allochthonousmaterial to document the impact of the 1945 Makran Trench earthquake in Pakistan on the shores of Oman.

As part of our study, Dr. Reinhardt and I pulled vibracore samples from the interior of Lechaion harbor for sedimentary and micropaleontological analysis. While the results of those cores has not yet been published, one conclusion seems certain. I was quite wrong in the 1995 and 1996 articles, which suggest the harbor went out of use quite early. All indications are that the harbor at Lechaion continued into use well in the 5th or 6th centuries AD (a conclusion also reached by the INQUA authors). I can’t explain the early date of the lithophaga shell, but we have numerous C14 dates from our cores that indicate otherwise. Our hesitancy in publishing this material has been driven by our increasing awareness of just how very complicated the palaeoseismic and archaeological records are for Lechaion. Lechaion is a shallow harbor in the Gulf of Corinth, and there are numerous phenomena that have impacted the record there, including harbor construction, coseismic uplift and sea-level change. Trying to isolate those causes from one another has proven most difficult.

I was very happy therefore to see that the 2011 INQUA volume contains two strong papers on Lechaion. Hadler et al.

conducted a study to identify “allochthonous high energy events” within the sedimentary record of the harbor. In simpler words, they pulled cores and looked for things that would not normally be found in calm harbor. When a tsunami occurs, even a small one, sediments and marine creatures normally found in deeper waters can be tossed inland. Donato’s work in Oman relied on a similar approach, and he identified tsunami deposits by locating articulated marine bivalves farther inland than they could have been carried by tidal activity or a storm surge.

When Reinhardt and I pulled the cores over a decade ago, tsunamis were not much in our thinking. In our 1999 investigation of the Izmit earthquake we documented a localized tsunami generated by submarine slumping of sediments. Hadler et al.note that the Gulf of Corinth is similarly susceptible to such events, and they focused their study of Lechaion on identifying these. The authors identify potential tsunamigenic impacts from the 8th century BC to the 6th century AD based on their vibracores. They also hypothesize that the gravel and marine material burying the Lechaion basilica is a result of tsunami backflow, ultimately suggesting a large 6th century AD tsunami.

They also suggest that beachrock in the area may be related to the tsunami.

Vibracoring (Lechaion) Fleur Leslie Hadler and colleagues make some interesting suggestions, but I am not fully convinced. The beachrock idea is new to me, and I am still researching that. The idea that the Lechaion basilica was buried by tsunami backflow I find least convincing, despite the added geophysical evidence of backflow provided by Koster et al. The harbor, beach, and basilica lie just above sea-level and are well within the reach of storm surges. In many years of working in the area I have seen storm surges cover and uncover archaeological features in and around the harbor.

Postulating a tsunami seems unnecessary. A few extremely large storm surges could create the same backwash and burying effect. The good news is that this is a “knowable unknown.” Careful excavation or coring of these sediments would allow grain-size analysis to be conducted. Variations in grain-size (or lack thereof) would very likely identify the number and timing of events responsible for burying the basilica.

I also think, however, that the archaeological record at the basilica is very complicated. Many visitors have asked the obvious while gazing upon the foundations of the folorn basilica: where is the rest of the building? The answer is, of course, all over.

The blocks and columns and walls were carted away for reuse. Disassembling and removing what was at the time the largest basilica in the world created a huge amount of debris and disturbance. I think some of the materials burying the basilica remnants are a byproduct of that work. But that also is something careful study could identify.

Hadler et al. also identify tsunamigenic elements in their cores. This I find more intriguing but complicated by the same issues holding back our study. The deposition of sediments in the harbor is controlled by multiple phenomena. The shallow harbor of Lechaion is particularly difficult for two reasons. The first is coseismic uplift. The entire coastline is being uplifted, and there has been uplift since the harbor was constructed. When uplift occurred, the harbor would have been isolated from the Gulf. Over time the topography of the gravel beach would have shifted from steep to level, and eventually a storm surge would push itself into the harbor. As uplift repeated, so would these events. But a second issue makes this even more complicated. The level of the Corinthian Gulf has risen about a meter since Roman times. Sea level rise has been gradual, and this is extremely difficult (if not impossible) to identify in sediment cores. Combine that sea level rise with uplift, storm surges, and possibly tsunamis and you have a very complex record indeed. This is one reason why the results of our study at Korphos, a Corinthian harbor that is subsiding, have already been published–the variables are easier to control when coastlines are going down than when they are going up. The tsunami hypothesis is intriguing, and gives us one more factor to consider.

Lechaion Basilica from the west, with dredge mound visible in distance In sum, the authors provide some much needed hard evidence in the consideration of the geological and archaeological history of the Lechaion harbor and basilica. Their data, if incorporated into a study that controls for all the environmental variables, will be a valuable contribution indeed. On a personal level, I also am happy to see independent confirmation of my idea that localized tsunamis are something we need to be considering. That idea took us into the heart of disasters in 1999-2001 and I like to think the difficulties were worthwhile.