The sea isn’t the steady thing we tend to think it is. It’s usually assumed that global sea level has been stable for around 7,000 years, i.e., throughout the existence of modern human civilization. We also assume that since the world’s oceans are interconnected, when sea level rises, it does so everywhere.
But the sea is a prankster, and closer study reveals shows local anomalies in its relative level – some of which beggar explanation. Now an international team of scientists headed by Assaf Yasur-Landau of the Leon H. Charney School of Marine Sciences, University of Haifa, reports in PLOS One on indications of such an anomaly on Israel’s Mediterranean coastline: an upward creep between the Mid-Bronze Age to Iron Age and then a sharp rise in the Hellenistic period, apparently by about 2.5 meters (8 feet) in total, to the level we know today. That is some anomaly.
During the Last Glacial Maximum about 26,000 years ago, the global mean sea level was about 126 meters lower than it is now because the vast ice sheets locked up the water. As the Ice Age waned and the glaciers blanketing the northern hemisphere melted, the global sea level rose to what we know today.
Let us not bog down on climate change-related sea level rise. That’s another story. So far, the mean global sea level has risen by 8 or 9 inches since 1880, according to the National Oceanic and Atmospheric Administration. The point is that, barring that 8 to 9 inches, sea level has been roughly unchanged on a global scale since the dawn of modern civilization.
But locally, 3,800 years ago, the sea level along the northern Israeli coast was about 2.5 meters lower than it is today, say Yasur-Landau and the team. The find is baffling: they cannot explain it, they admit.
The sea level change didn’t happen overnight, Yasur-Landau adds. “We are talking about things that happened quite slowly between the Bronze and Iron Age,” he tells Haaretz.
Then, between the Hellenistic and Roman periods, the local sea level rose quickly, to its present level (minus climate change’s 8 to 9 inches). They can’t explain that either.
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Driving home the startling magnitude of the local change, Dr. Thomas Levy of the University of California San Diego, who participated in the research, noted that a sea level rise of such magnitude is unknown in the historic period: the last time the seas rose drastically was at the end of the Ice Age. Today, the low end of forecasts speaks of mean global sea level rising by a meter by 2100; in this case we see double that, he points out.
One convenience when investigating historic life by the sea is the nature of the Mediterranean along the Israeli coast: it is micro-tidal. The tidal amplitude is about plus-minus 30 to 40 centimeters. The Mediterranean doesn’t have “king tides.” So tides aren’t a factor in this story.
How were the sea level anomalies detected? On coastlines, archaeological finds of settlements along the shoreline are accepted as proxies for reconstructing sea level during the Holocene, the current geological epoch that began around 11,500 years ago as the glaciers retreated.
The balmy micro-tidal Israeli coast is and was thronged with people since time immemorial, leaving behind a rich archaeological record. Remains of coastal villages from as long as 10,000 to 7,000 years ago have been discovered beneath the waves in Israel and Europe: they became submerged as the ice melted. But after that, sea level was supposed to be stable. No?
The hypothesis of low relative sea level from Bronze Age and a rapid rise into the early Roman era is based partly on finds at Tel Dor, an ancient port on the Mediterranean coast south of Haifa that was occupied from the Middle Bronze Age through to the Byzantine period (the fifth century), a span of over 2,200 years. What did the archaeologists find there?
How to dismay a Roman
Like many archaeological sites, Tel Dor features stratification of the ages. Its earliest seawall went up in the Iron Age, in the late 12th or early 11th century B.C.E. Then, in the 11th century B.C.E., the inhabitants erected massive coastal fortifications atop that early seawall, the archaeologists found. In the seventh century B.C.E., around 2,700 years ago, in the Assyrian period, a massive sea gate was built atop the stone foundations of the fortifications.
First of all, what the archaeologists found is that the foundations of the Iron Age fortification are under water. It would have been on dry land when it was built.
Second, they “found” something missing: the Romans did not build any marine facilities or fortifications of their own there, though the settlement remained inhabited. The assumption is that if the convenient port area hadn’t been submerged by their time, they would have.
Third, apropos the Romans, they found that a pool connected with the sea that they did build there later continues to receive water from the sea. This indicates that by then, the sea had reached the level it is today. But structures from the Hellenistic period at Dor and other coastal sites indicate that sea level had been similar to that in the Iron Age: low.
Fourth, numerous smaller coastal settlements clearly declined or collapsed outright during the Hellenistic period, among them Yavneh Yam, Ashdod Yam, Straton’s Tower and Tel Taninim. Parts of them are now under water or at the waterfront.
Yavneh Yam evinces a decline in settlement of the acropolis between the Hellenistic and Early Roman periods, as the sea was rising anew, though life went on and farming continued north of the site. In Ashdod Yam, the monumental Hellenistic structures on top of its acropolis were destroyed in the late second century B.C.E. and were not rebuilt by the Romans; Straton’s Tower (later Caesarea) “probably” featured elaborate harbor facilities in the early Hellenistic period, but was abandoned by the time of the Romans. In Akko-Ptolemais, the Hellenistic port facilities were submerged, then allowed to decay during the Roman period, the team explains.
The team believes that sea level rise between the Hellenistic and Roman times degraded the infrastructure. Some of the towns didn’t drown, they slowly starved. Weakened, they were unable to bristle properly when the brutal Hasmonean and Roman expansion campaigns began.
Why didn’t the construction-mad Romans rebuild these port facilities? “With the relics of earlier installations still protruding dysfunctional out of the water, knowledge of the rising sea level and its rapid pace would have been widely available, serving as a painful reminder of past glory for such cities as Dor and Akko [aka Acre], and as a clear discouragement of investment in coastal facilities for the entire area,” explains Dr. Gil Gambash from the University of Haifa, who participated in the research.
In separate work, while diving at Tel Dor, archaeologists also recently found another stone anchor on the seabed there. Since Tel Dor was serially colonized by everyone from the Sea Peoples to the Romans, its seabed is littered with antiquities from the shipping trade and/or wrecks. These rocks that had served as ballast could not have originated locally – they had to have been in the bellies of foreign boats, archaeologists explain.
But we digress. What could have caused these sea level anomalies?
“Relative sea level is one of the stranger phenomena. In one place it can drop and in another it can rise,” Yasur-Landau remarks.
For instance, glaciers covering a land mass weigh it down; when the glaciers melt, the land experiences post-glacial rebound. Relieved of the weight, it gradually rises relative to the sea level. Parts of North America are still responding to the Ice Age’s end.
A reverse effect plaguing coastal lands nowadays is sinking relative to the sea, because of sea level rise, because the groundwater is being overexploited and depleted; and in the case of the big cities, because the sheer weight of the buildings is compressing the ground. This is why science speaks of “global mean sea level” – which is slowly rising with climate change, as contrasted with “relative sea level,” which is a local thing. Jakarta, London and New York are sinking; Scandinavia is rising.
But glacial rebound can’t explain why the southern Levant seemingly rose relative to the sea here in the Bronze Age, nor obviously can it explain why the sea seemingly rebounded fast in the Hellenistic and early Roman era – a discovery made by this team.
“This is the first study of sea level changes from the Iron period to the present in our time,” said Prof. Dorit Sivan, who co-led the research.
To be clear, Tel Dor is far from the only localized anomaly in Mediterranean Sea level. The team points to a similar and equally inexplicable find in France: sea-level proxy observations between 4,000 to 3,000 years ago indicate local levels that were 1 to 2 meters below present levels. Long story short, the situation on part of the French coast seems similar to the Israeli case. Moving onto Corsica, in the island’s northern coast, proxies indicate that the relative sea level was more than a meter lower around 3,700 years ago than today. By the Roman period, it had rebounded somewhat but was still half a meter below present level.
Day of horror
All other constraints aside, comparison can’t be made with the coasts of Lebanon and Syria because they’re tectonically active, the team adds. The Lebanese coastline north of the Rosh Hanikra (Ras Nakura) fault line has been marked by vertical activity during the Holocene. So, across the Mediterranean, was Crete. What about here?
“There are a lot of mechanisms that can affect sea level rise,” Yasur-Landau points out. “It could be that in the east Mediterranean, something happened.”
Must have, really, but we don’t know what it was.
As for Crete, on the other side of the Mediterranean bathtub, in the year 365, there was a massive earthquake that lifted parts of the island by several meters. A recent study by the GFZ German Research Centre for Geosciences, published in AGU Advances, pinpointed the origin of that temblor, which was remembered for centuries as a “day of horror,” explains the GFZ’s Richard Ott. It may have been the most powerful quake in the recorded history of the Mediterranean and triggered tsunamis that washed all over the region’s coasts, of course reaching Israel too, he tells Haaretz.
But Crete sits smack on a tectonically active part of the crust while Israel’s side of the Mediterranean coast is tectonically inert, Yasur-Landau explains. We had no quake here that could have lifted the land, or vice versa. It bears adding that the Dead Sea Transform faults are responsible for some hefty earthquakes throughout Israel’s history, but these are inland.
So quake as an explanation is apparently out. Asked if they have any other hypothesis that could explain their strange find, Yasur-Landau cheerfully says they do not. “It will take us time to figure out the mechanism,” he says, but notes: “Archaeology is the strongest indicator of this sort of thing.”