Icy ebb and flow influenced by hydrothermal activity
The last million years of Earth's history was dominated by the cyclic advance and retreat of ice sheets over large swaths of North America. During cold glacial intervals, ice sheets reached as far south as Long Island and Indiana, while during warm interglacial periods the ice rapidly retreated to Greenland. It has long been known that ice ages occur every 40,000 years or so, but the cause of rapid transition between glacial and interglacial periods has remained a mystery.
While conventional wisdom says that this icy ebb and flow is an interaction between the world's oceans, the ice itself, and the earth's atmosphere, an article appearing in the Jan. 28, 2016 issue of the journal Science sheds new light on the role that the earth itself may play in this climatological ballet.
David Lund of the Department of Marine Sciences at the University of Connecticut and his colleagues have studied hydrothermal activity along the mid-ocean ridge system — the longest mountain range in the world which extends some 37,000 miles along the ocean floor. Their research suggests that the release of hot molten rock, or magma, from beneath the earth's crust in response to changes in sea level plays a significant role in the earth's climate. This change is attributed to the release of heat and carbon dioxide (CO2) into the deep ocean.
Lund says, "Mid-ocean range magmatism — the release of molten rock through volcanic vents or fissures — is driven by seafloor spreading and decompression melting of the upper mantle" — the partially molten layer just beneath the earth's crust.
"This activity is controlled by the rate of pressure release at any given location. There's clear evidence that when ice sheets grow, sea level lowers and significant pressure is taken off the ocean ridges. This causes melting in the mantle, which should in turn promote the release of heat and carbon into the oceans — and that's when glacial termination begins — meaning the ice starts to melt. Then, sea levels begin to rise, pressure on the ridges increases, and magmatic activity decreases."
Well-documented sedimentary records from the East Pacific Rise (EPR) — a mid-ocean ridge extending roughly from Antarctica to the Gulf of California — show evidence of enhanced hydrothermal activity during the last two glacial terminations, the last of which took place about 15,000 years ago.
According to Lund, the southern East Pacific rise (SEPR) has the fastest spreading rate and the highest magmatic budget of any ridge in the global mid-ocean ridge system. Due to its elevated magmatism, the SEPR has over 50 known active vent sites.
He says, "The coincidence in timing between hydrothermal maxima and glacial terminations implies that there may be a direct causal relationship between hydrothermal activity and deglaciation … Our results support the hypothesis that enhanced ridge magmatism, hydrothermal output, and perhaps mantle CO2 flux acts as a negative feedback on ice-sheet size … "
In this study, core samples from both sides of the ridge axis were analyzed and included radiocarbon and oxygen isotopic analyses of microscopic shells to provide age control for each core. Major and trace element concentrations were determined using x-ray florescence and inductively coupled plasma mass spectrometry.
The EPR results establish the timing of hydrothermal anomalies, an essential prerequisite for determining whether ridge magmatism can act as a negative feedback on ice-sheet size.
David Lund,, Dept. of Marine Sciences, UConn