Over the past two months, the DUSTCO team has joined Patrizia Ziveri at ICTA-UAB Unit of Excellence, in Barcelona (Spain), to work on sediment trap material collected along our transatlantic transect. Patrizia is our scientific partner supporting the coccolithophore biogeochemical aspects of the project. In addition to being Scientific Director at the ICTA-UAB, she is one of the leading experts in this field of research with an extensive and world-wide experience in the coccolithophores seasonal export production and related interaction with biogeochemical cycles.
Patrizia Ziveri, together with Heather Stoll, did pioneering work on applying the coccolith-Sr/Ca ratio as productivity proxy, which is precisely what brought us here. Our plan is to measure the coccolith-Sr/Ca from 24 samples equivalent to 1-year (2012-2013) of coccolith export fluxes at trap site M4 (49ºN 12ºW), where we identified the occurrence of dust fertilization during spring and fall (see Guerreiro et al., 2017). With this biogeochemical analysis, we want to be able to distinguish Sr/Ca ratios from “background” tropical conditions vs. Sr/Ca ratios influenced by dust-born nutrient input. We are especially curious about the fast-blooming species E. huxleyi and G. oceanica, since they were the fastest responding to dust in the Atlantic, about the deep-dwelling species F. profunda and G. flabellatus which were dominant throughout the year by appeared unaltered by dust. Our ultimate aim is to use the Sr/Ca-coccolith ratios as a proxy for distinguishing dust-related productivity from dust-related mineral ballasting.
These are the sediment trap samples from our site M4, located at the western tropical North Atlantic (12ºN/49ºW). Each sample represents ~16 days off particle sinking from the surface of the ocean all the way down to 1200 m depth, where the trap was moored for one entire year. This picture clearly illustrates how the color of the sinking particles changed throughout the year, resulting from changing environmental - meteorological and/or oceanographic conditions in the study region These include the sporadic deposition of dust blown all the way from Africa in April and October-November 2013, and the inflow of the Amazon River Plume flowing all the way from NE South America during the fall of 2013 (see Guerreiro et al., 2017).
This is Patrizia Ziveri, checking the preservation and composition of the coccolith assemblages in the suspensions resulting from the repeated decanting, using a light microscope.
The laboratory procedure was long and fastidious! We started by resuspending and disaggregating the sediment trap samples with a buffered solution, after which each sample solution was split into 10 aliquots using a rotary splitter. Three of these splits were mixed together and wet-sieved for obtaining three distinct particle-size fractions: 150-63 mm, 63-20 mm and <20 mm. From the remaining splits, five of them were also mixed together and wet-sieved for obtaining the <20 mm fraction, which was then subjected to several cycles of repeated decanting for gravitationally separate large-, intermediate- and small-sized fractions of coccoliths.
All decanting experiments were executed by allowing suspensions to settle in 50-ml centrifuge tubes and pipetting off the desired fractions of supernatant, using the Stokes Law as an approximation for the relative settling times of different-sized particles (e.g. Stoll and Ziveri, 2002).
So far, we are done with preparing all six size-fractions per each sediment trap sample – meaning a total of 138 filters! These will be further chemically analyzed, using an inductively coupled plasma-mass spectrometry on a Finnigan ELEMENT sector field instrument, for determining the amount of Ca and the Sr/Ca ratio.
We want to thank Patrizia for hosting us at ICTA-UAB during this intensive secondment of the project DUSTCO. We look forward to the interesting biogeochemical discussions that await for us in the following months, once we puzzle all the results together!
