16 December 2009

Emiliani Lecture and the Tough Task of Paleoceanographers

Posted by Michael McFadden

The Emiliani lecture (PP31E), given in honor of one of the fathers of the field of paleoceangraphy, Cesare Emiliani, draws just about everyone at AGU who’s ever thought about paleoclimate, even if it was just “huh, paleoclimate and paleooceans are cool.” This year,  Delia Oppo, from Woods Hole Oceanographic Institute was the guest lecturer, presenting on “Holocene Changes in the Indonesian Throughtflow region.”

I’m a student, and I’m taking my paleoceanography final exam tomorrow (yes, here at the meeting). Over the course of the semester I’ve been assigned a whole bunch of her older papers; it seemed like a good idea for me to go see what she had to say. I would get to review for my exam and learn about some interesting science, all at once.

Sediment cores taken from ocean floor beneath the Indonesian Throughflow region.  Image from D. OppoDr. Oppo and her colleagues studied forams–tiny, microscopic shells–from sediment cores (as seen on the right), and used oxygen isotope ratios to infer how different water masses—the cooler, fresher South China Sea, and fingers of the warmer, saltier Indian Ocean—traveled through several Indonesian straits during the late- and mid-Holocene. They expected to see dramatic changes in cycling between the late Holocene, with its frequent El Nino events (which would cool and freshen the South China Sea, which would then slow down transport through the major strait they looked at) and the mid-Holocene, which had conditions frequently more like a La Nina event.

Their hypothesis was very clearly not supported by observations! They didn’t see any extra influx of northern or Southern water masses through their major strait, and they didn’t see evidence of any major circulation changes between the late and mid-Holocene. Instead of scrapping the project, though, they worked through alternative hypotheses, and in doing so found that changes in insolation (solar input) between the late and mid-Holocene basically decreased the between-season variability of the region, masking whatever changes in the ENSO-like cycling might have happened at the same time.

The project, of course, was fascinating—there are so many different factors at work in the Indonesian region that picking apart causes and effects seems like an insane project, but the fact that they dug into the problem is why they’re doing the research, not me! What I appreciated seeing most of all, though, was a frank discussion of how their original hypothesis didn’t work out. One of the inherent things about paleoclimate research is how incomplete our understanding of mechanisms is—we know a lot, but there is always more to learn. It’s really valuable to find out that an oceanographic system doesn’t work like we thought it would, as long as we try to figure out what actually is going on. Seeing an eminent paleoceangrapher discuss revisions to her original ideas is refreshing.

It’s back to studying for me. Dr. Oppo’s lecture got me up to speed on Western Pacific ENSO cycling effects and orbital variability throughout the Holocene, but there’s the whole rest of the globe to review before I can take my exam!

–Ale Borunda, Columbia University Earth and Environmental Sciences Journalism Graduate Student