WHOI paleoceanographer Sophie Hines is fascinated by abrupt climate change—how the Earth’s climate system shifts in abrupt and unexpected ways. A former Lamont Postdoctoral Fellow at the Lamont-Doherty Earth Observatory of Columbia University, she studies rapid changes in ocean circulation that occurred during pivotal transitions in the Earth’s climate—long before the modern ocean.
Oceanus caught up with Hines to learn how one of her primary go-to paleo archives— deep-sea fossil corals—offer clues about past climate and circulation patterns which in turn can help us understand what might happen in the future.
Oceanus: Your work as a paleoceanographer would seem to give you a really broad view of climate change over the span of millions of years. Why do you find the smaller, more abrupt changes in climate so compelling?
Hines: Abrupt changes, particularly in between the Last Glacial Maximum (21,000 years ago) and the Holocene (10,000 years ago) when the modern ocean circulation came into play, offer incredible snapshots in time of what was going on in the ocean, and how those events influenced Earth’s climate in the past. For example, we’ve found through analysis of deep-sea fossil coral archives that abrupt changes in ocean circulation have contributed to changes in global temperature and precipitation. And we’ve seen pretty compelling evidence of big circulation shifts from corals. For example, some of the samples we’ve analyzed tell us that the Atlantic Meridional Overturning Circulation, which regulates global climate, shut down for a period of time during the deglaciation and then started back up really abruptly.
Oceanus: Paleoceanographers have a range of archival tools they can use to look back in time—sediment cores, ice cores, etc. Why do you choose deep-sea corals as your “time machine” of choice?
Hines: Deep-sea corals are unique amongst paleo archives in that it’s really easy to get very precise information on their age. Unlike sediment cores, where establishing an age chronology is a pretty difficult task, corals can be directly dated so they have a much better age constraint, And, they have a unique ability to provide information about abrupt changes since they continuously record information about the water they’re growing in. This includes changes in the ocean’s oxygen concentration, acidity of the seawater, and circulation. We can understand these changes by measuring geochemical tracers, the most common being radiocarbon which is produced in the atmosphere before it sinks into the ocean and decays over long time periods. It therefore tells us about deep ocean circulation patterns and how quickly water moved through the deep ocean.
The other advantage these fossil corals provide is their size—most of them are pretty big so you can measure a number of geochemical proxies on a single sample. This allows you to simultaneously get information about deep ocean temperature and circulation rate.
Oceanus: Where are your coral samples from, and how did you get them?
Hines: Most of the samples I’ve analyzed come from the New England Seamounts and I’ve also used corals from south of Tasmania. We typically collect them with a submersible like Jason or Alvin and scoop them up from the seafloor and put them into buckets attached to the submersible. Knowing the precise ocean depths the samples are collected from is an important first step towards reconstructing past ocean changes.
Oceanus: What are some of the ancient ocean’s most dramatic changes you’ve seen through these records?
Hines: The deglaciation, following the Last Glacial Maximum, around 21,000 years ago, is when we see the most dramatic period of climate change before human activities. It involved big changes in the amount of CO2 in the atmosphere, which was being released in vast quantities from the deep ocean at the time. Before then, there had been more stratification in the deep ocean which allowed more carbon to be stored. But as stratification became less stable, particularly in the Southern Ocean, there was suddenly more deep water mixing with shallower water and this helped bring carbon out of the deep and into the atmosphere. These circulation changes affected climate by causing a dramatic warming interval, which you can see in the water that the corals were growing in at the time.
One thing that’s been really intriguing is that when we take radiocarbon and temperature data from these ancient deep-sea corals and compare them with radiocarbon and temperature levels in the modern ocean at the same locations, they don’t match any values we see in the ocean today. That suggests to us that ocean circulation was fundamentally different back then.
Oceanus: Fundamentally different how?
Hines: During these big climate changes, it appears as though ocean circulation had been reinvigorated as CO2 is coming out of the ocean and younger water begins to displace old water in the deep.
Oceanus: Your work provides a unique window into a lot of natural climate variability during last Ice Age. Does that help in projecting future changes to our modern climate?
Hines: I believe it does. The amount of CO2 in the atmosphere is much different than what we’ve seen in the past—it’s unprecedented. We can use clues from some of the natural variability that we’ve seen in the Earth’s climate system to try to understand how we think the ocean circulation might change if, for example, there’s a big change in ice sheet melting today. There are big natural changes that we can trace from the past, and you could surmise about how the future ocean would change based on our knowledge of those natural changes.