The formation of Hachirogata
Hachirogata is believed to have become a brackish lake over 2,000 years ago, after having been both sea and land in earlier times.
Over the past 2.6 million years (the Quaternary period), Earth’s climate has fluctuated significantly.
During colder periods, water evaporated from the sea and accumulated as ice in the polar regions, lowering sea levels.
When the climate warmed, ice melted and returned to the sea, causing sea levels to rise.
These fluctuations are thought to continue today.
The formation of Hachirogata was closely linked to these sea level changes over the past 10,000 years.
(Note: The term “Hachirogata” here refers to the area before it became a brackish lake.)
*Brackish water: Water with lower salinity than seawater.
-About 10,000 years ago-
20,000 years ago, the climate was much colder, and sea levels were more than 100 meters lower than today.
As the climate warmed, sea levels gradually rose.
However, 10,000 years ago, they were still tens of meters below their current levels.
As a result, the Oga Peninsula was connected to the mainland, and Hachirogata was dry land.
-About 8,000 years ago-
As sea levels continued to rise, the sea gradually flooded the lower-lying areas.
This transformed Hachirogata into a deep bay, where Pacific oyster thrived in large numbers.
While sea levels had not yet reached their present height, it is believed that sea temperatures were already higher than they are today.
-About 6,000 years ago-
As sea levels rose further, Hachirogata became a shallow sea between the Oga Peninsula and the mainland.
The Oga Peninsula was cut off from the mainland and became an island.
With rising sea temperatures, species from the southern seas, such as chirimen yukigai (Meropesta sinojaponica : a type of clam), moved northward and began to inhabit Hachirogata.
-About 4,000 years ago-
As the climate cooled and sea levels dropped, sand spits formed on the sea floor in the previous era began to emerge above the water, causing Hachirogata to become separated from the Sea of Japan.
The water flow slightly worsened, and chiyonohanagai (Raetellops pulchellus: a type of clam) increased in number.
Around this time, much of the land around the central area was still above water.
sand spits: A long, narrow rise formed from sand and other materials carried by water currents.
-About 2,000 years ago-
As the sand spits continued to develop, the influx of seawater from the Sea of Japan into Hachirogata decreased.
The water became more diluted by river and rainwater, lowering its salinity, which allowed yamatoshijimi (Corbicula japonica ) to I ncreased.
Eventually, the northern waterway was completely blocked, and Hachirogata was fully formed.
