Ocean Mud, Ocean Burping, and Why the Climate Could Change Abruptly

Earth's story is told in ocean mud... Sarah Myhre

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Did you know that Earth's history is written in the texture, composition, and chemistry of mud from the deep sea? That's right—deep-sea sediments are exquisite climate and environmental archives. They hold some, if not the most, important information about how the ocean and climate system have functioned in the past, and how it will function in the future. Ocean mud and climate go together because oceans are reservoirs for vast, unthinkable quantities of carbon. In the deep ocean of this one finite planet, there are 37,000 Gigatons (Gt) of carbon (C), dwarfing the carbon retained by both the atmosphere (600 Gt) and all terrestrial soils and vegetation combined (2,300 Gt). The ocean is the carbon and climate system behemoth. So, if you want to understand the carbon system and climate of the past, you have to look at ocean sediments.

Understanding our past is the key to the future

Recent Earth history is characterized by oscillations between glacial and interglacial climates—the heartbeat, if you will, of the planetary system. Eight glacial cycles have occurred in the last 740,000 years, and we describe these glacial climates as having a "sawtooth" pattern: rapid warming (into warm interglacial climates), followed by gradual cooling (into cold glacial climates). This pattern matters. We know from Pleistocene climate archives that the Earth system is sensitive to tipping points that produce abrupt (less than 100-year) warming, but it takes tens of thousands of years to gradually cool the system.

Stepping back, you might ask, “what fundamentally produces a glacial-interglacial pattern in climate variability?” This is where things get cool. These climate shifts are a product of astronomical climate variability and the changing geometric relationship between the Earth and the sun. These wiggles and wobbles to Earth's orbit and rotation produce predictable and observable patterns in how solar energy is distributed on Earth's surface.

But Earth’s orbital cycles are relatively long, with 23,000, 40,000, 100,000 year frequencies of variation (these wobbles are called, respectively, precession, obliquity, and eccentricity). Why then do we see abrupt changes in glacial/interglacial climates? This, again, is where the carbon system comes in. Small shifts in the latitudinal distribution of solar energy destabilize glacial ice sheets—and the catastrophic collapse and shedding of continental ice alters huge carbon reservoirs, producing infamous “positive feedbacks,” which amplify abrupt warming through the addition of greenhouse gasses into the atmosphere.

So, short-term changes in climate in the past matter. A lot. Because these events demonstrate pieces of the Earth system (i.e. ice, ocean circulation, volcanoes, ice sheets) that are connected to shifts in the carbon system. One of my favorite positive feedbacks in the system is the idea that the deep ocean “burps” enormous amounts of carbon into the atmosphere, through the upward mixing of deep ocean waters, during abrupt warming events. Another positive feedback that I enjoy is the idea that catastrophic ice shedding, and the resultant unweighting of the continental crust, causes volcanic eruptions, which emit volcanic-sourced carbon into the atmosphere. These examples scratch the surface of how ocean sediments allow for the investigation of past changes in the carbon system. They are vital for understanding celestial mechanics and Earth system feedbacks.

Despite the expense of recovering these samples and the importance of the carbon-ocean system, there is no common database to archive and interpret climate information extracted from the deep sea. There are physical repositories, such as the one at Oregon State University, where giant walk-in refrigerators store thousands of sediment cores in plastic sheathes. There are also online federal data portals. But cores are, by-and-large, published individually, on a piecemeal basis–and this should actually demonstrate how we as a field are moving from a data-poor to a data-rich stage. We are making that transition now.