Juliet Sefton is a first year physical geography PhD student studying mangrove sediments and how they can be used to reconstruct past sea levels. Here, she reports on her first field trip for her PhD research to the mangrove forests of the Seychelles in July 2017. Follow her on Twitter @SeftonGeo for more updates. 

Reblogged from the Climate Research Stories at Climatica 

Dawn had already broken when our plane touched down on Mahé island, lighting up the cloudy, shifting sky and the turquoise waters lapping at the runway’s edge. From my plane window I could already see sheer cliffs of pinky-grey granite rising up from the narrow coastal plain, covered in lush tropical vegetation. I was sleepy, but excited to be here. Mahé is the largest island of the small island nation of Seychelles – an archipelago in the western Indian Ocean. I was visiting for three weeks to begin the field work component of my PhD research. My destination: the mangrove forests that fringe the western side of the island.

Mangrove forest at Barbarons, on the western side of Mahé island

Mangrove environments occupy a narrow coastal zone between high and low tide. These environments are dominated by mangrove trees – species that have evolved to tolerate frequent flooding by salty, marine waters as the tides wax and wane. Mangrove trees can be fussy – some species like to be flooded more often than others, and they tend to position themselves accordingly. As scientists, we can get a sense of this distribution by observing the vegetation patterns across a mangrove environment. This fussy nature means mangroves are sensitive to changes in sea level over time, and these changes can be recorded in the sediments beneath the mangrove trees. My PhD involves examining these mangrove sediments – to understand what parts of the sediment best inform us about past sea-level changes.

Wading in the mangrove forest

We drove the winding, cracked roads up over the mountainous centre of Mahé to a mangrove occupying the mouth of the Dauban River, near the village of Barbarons. Over the following weeks, Sarah Woodroffe (my PhD supervisor), Richard Jones (field assistant) and I were squelching about in the mangrove mud and roots collecting field observations and sediment samples. Our assumption is that as sea level rises and falls, mangrove species will shift up or down the environment in response. By sampling the modern surface sediments across the mangrove environment, we can better understand shifts we see in sediment cores, recording environmental changes over time.

Collecting sediment cores in the upper zone of the mangrove forest

I soon discovered that working in a mangrove isn’t a glamorous as I thought it was going to be. While Seychelles has undoubtedly some of the most exquisite white-sandy, coconut palm-fringed bays, the mangroves themselves are a different story. Even in the heat and humidity of the tropics, we kitted-up in full boiler suits and DEET to keep nasties and the mud at bay. In the mangrove canopy, golden potter wasps make their nests, and large palm spiders (ugly, but harmless) weave their webs across the branches, perfectly at head-height. Hungry mosquitoes thrive in the damp, shady mangroves, and stinging hairy caterpillar grubs drop down from the leaves above. We had to keep an eye on our watches, and make sure we had an escape out of the mangroves as high tide encroached and flooded our sampling areas.

Surveying at Barbarons

While we didn’t enjoy the company of some our wildlife neighbours, there were others that were more exciting to come across. If you stayed still for a moment, the ground surface would erupt with movement of multiple species of brightly-coloured crabs, all fighting for a leaf or crumb of our sandwiches to scurry away into their holes with. Bright green geckos scampered up tree trunks, and we were constantly followed by birdsong. Guppies, hermit crabs, snails and juvenile reef fish could be seen swimming amongst the submerged mangrove roots.

Palm Spider

Red Mangrove Crab

Green Day Gecko

As well as collecting sediment samples, we also deployed traps to collect pollen with the help of the Seychelles National Park Authority. This is to better understand how mangrove pollen is transported to the sediment surface, and these will be left to collect material for about a year. After some successful sampling, the material was all boxed up, and checked-in as our baggage home to the UK. Grateful for escaping the muddy mangroves but sad to leave tropical island paradise, we boarded our flight back to a drizzly and cold Newcastle… until next time!

Collecting sediment surface samples and racing the incoming tide!

Now, laboratory work begins – starting with preparing sediment for geochemical analyses. An objective of my research is to explore new methods to reconstruct past sea level from mangrove sediments. The molecular remains of mangrove organic matter (called biomarkers) can be used to understand past salinity (salt concentrations), even though the leaves and roots the molecules come from have decomposed away.

It will be many months before I have any further results, but I’m ready to get cracking. To be continued!

 

Below is a short video produced by Richard Jones (a post-doctoral researcher here at Durham, Twitter @selwynox). It shows some great footage of what it’s like to work in a mangrove forest, and myself trying to explain what I’ve been up to! 

 

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