USGS Multimedia Gallery: (Video)--What Lies Beneath: Using Mangrove Peat to Study Ancient Coastal Environments and Sea-Level Rise

McKee: We‘re on a mangrove island off the coast of Belize, and we’ve come here to collect some cores

through the entire profile of the island down to the Pleistocene limestone. Beneath me is eleven meters

of peat. That’s the equivalent of a three- story building.

What we are doing is we are collecting cores in segments that we’re bringing up and collecting and

preserving and later we’re going to be looking at the botanical fossils that are contained in those cores.

We’ll also be radiocarbon dating some of these cores to tell us how old each segment of the core is.

Previous cores suggest that this island has always been a mangrove system and it has built vertically

over a period of about 8,000 years…as sea level has risen over that time period. It has built vertically

many, many meters and has accumulated this peat very gradually. And this allows the island to keep up

with changes in sea level.

Vervaeke: Ready?

McKee: Yep.

Vervaeke: Got it?

McKee: Yep. OK.

McKee: OK, so this is one of our peat cores. It’s a half-meter section. This is from four point five to five

meters in depth, and this is probably between forty-five hundred and five thousand years old. And what

we’ll be doing is we’ll be collecting this and taking it back to the lab where we will log in all the

characteristics of it…anything…any strata..sections that we see in this…any changes in the consistency of

the core. Then we’ll be taking subsamples to identify any botanical fragments that we find in this core.

And we’ll also be taking samples for looking at the chemistry of the core as well as some other features

that will tell us something about the past environment that this peat was formed under. So now we’re

gonna package up this core and take the next section.

McKee: OK. So, we’re at ten meters..here. Still in mangrove peat. Pretty solid. I can see a lot of root

material and some leaf material.

Vervaeke: Count us down, Karen.

McKee: OK. One. Two. Three. Holy smokes!

Vervaeke: There it is!

(laughter)

McKee: Alright!

Vervaeke: Ha, ha..good call.

McKee: Okay…

Vervaeke: Alright.

McKee/Vervaeke: (gasp) Ooohh. Look at that.

Vervaeke: Keep going?

McKee: (gasp) Look at that. (Gasp)

Vervaeke: What do you want me to do?

McKee: Just hold it.

Vervaeke: OK.

McKee: Uh. Wow.

Vervaeke: What is it?

McKee: It’s, ah, sediment. Clay…it looks clay-like.

Vervaeke: That’s new.

McKee: Yes.

Vervaeke: Got it?

McKee: Yep .

McKee: I’m here in the Smithsonian’s Carrie Bow Cay field laboratory, and what I’m doing is examining

under magnification the peat samples that we collected yesterday. By identifying fragments within the

peat, this will tell us something about the plants and plant communities that existed in the past in this

mangrove ecosystem. We take photographs of these fragments, and compare them to modern material

and this allows us to identify the species of plants that were present in the past.

McKee: With dating of the peat, we can use those dates to reconstruct the sea level history of this

region along with cores taken in other locations around the Caribbean.

McKee (animation narration): By radiocarbon dating peat at different depths, we can determine how

many years ago that peat was formed. This core covered a time span of almost 8,000 years…based on

the oldest date measured at the bottom of the core. This time period is referred to as the Holocene.

Beneath the Holocene peat is limestone that was formed during the Pleistocene, an earlier time period.

By comparing the peat dates with a sea-level curve for the region, we can see if this mangrove island has

kept pace. The four peat dates measured in this core all fall on the sea-level curve, which shows that

these mangroves have kept up with sea-level rise for the past eight thousand years.

These mangrove islands accomplish this feat by slowly accumulating roots in layers at the soil surface.

Over time, the roots die and contribute to peat formation.

Because oxygen is lacking in the saturated soil, the roots are preserved, creating a strong matrix that is

not easily eroded. These fossil roots were dated and found to be over 7500 years old.

McKee: What this work tells us is that intact and healthy mangroves are essential to vertical building of

these islands as sea level increases. The information from this project will also help us understand how

mangroves may respond to future sea level rise.

IMAGES AND ANIMATIONS

“Mangrove Peat Island”, “Peat Chronology”, and “Peat Formation” animations; videography; and still

photography: Karen L. McKee

MUSIC AND SOUND EFFECTS

“Jaracanda”, “Island Long”, “Pendulum” (Apple iMovie Royalty-Free Music Clips)

REFERENCES

McKee, K.L. 2004. Belowground dynamics in mangrove ecosystems. USGS Fact Sheet 2004-3126.

McKee, K.L. 2004. Global change impacts on mangrove ecosystems. USGS Fact Sheet 2004-3125.

McKee, K.L. 2011. Biophysical controls on vertical accretion and elevation change in Caribbean

mangrove ecosystems. Estuarine, Coastal and Shelf Science 91: 475-483.

McKee, K.L., D.R. Cahoon, and I.C. Feller. 2007. Caribbean mangroves adjust to rising sea-level through

biotic controls on change in soil elevation. Global Ecology and Biogeography 16: 545-556.

McKee, K.L. and P.L. Faulkner. 2000. Mangrove peat analysis and reconstruction of vegetation history at

the Pelican Cays, Belize. Atoll Research Bulletin 468: 46-58.

Middleton, B.A. and K.L. McKee. 2001. Degradation of mangrove tissues and implications for peat

formation in Belizean island forests. J. Ecology 89(5): 818-828.

Toscano, M.A., & Macintyre, I.G. 2003. Corrected western Atlantic sea-level curve for the last 11,000

years based on calibrated C-14 dates from Acropora palmata framework and intertidal mangrove peat.

Coral Reefs 22: 257-270.

Rutzler, K., & Feller, I.C. 1996. Caribbean mangrove swamps. Scientific American 274: 94-99.

Filmed on location in Belize.

Thanks to the Smithsonian Institution’s Carrie Bow Cay Laboratory for logistical support.