Testing the D / H ratio of alkenones and palmitic acid as salinity proxies in the Amazon Plume

Abstract

Abstract. The stable hydrogen isotope composition of lipid biomarkers, such as alkenones, is a promising new tool for the improvement of palaeosalinity reconstructions. Laboratory studies confirmed the correlation between lipid biomarker δD composition (δD Lipid ), water δD composition (δD H 2 O ) and salinity; yet there is limited insight into the applicability of this proxy in oceanic environments. To fill this gap, we test the use of the δD composition of alkenones (δD C 37 ) and palmitic acid (δD PA ) as salinity proxies using samples of surface suspended material along the distinct salinity gradient induced by the Amazon Plume. Our results indicate a positive correlation between salinity and δD H 2 O , while the relationship between δD H 2 O and δD Lipid is more complex: δD PA correlates strongly with δD H 2 O (r 2 = 0.81) and shows a salinity-dependent isotopic fractionation factor. δD C 37 only correlates with δD H 2 O in a small number (n = 8) of samples with alkenone concentrations > 10 ng L −1 , while there is no correlation if all samples are taken into account. These findings are mirrored by alkenone-based temperature reconstructions, which are inaccurate for samples with low alkenone concentrations. Deviations in δD C 37 and temperature are likely to be caused by limited haptophyte algae growth due to low salinity and light limitation imposed by the Amazon Plume. Our study confirms the applicability of δD Lipid as a salinity proxy in oceanic environments. But it raises a note of caution concerning regions where low alkenone production can be expected due to low salinity and light limitation, for instance, under strong riverine discharge.

Effects of alkalinity and salinity at low and high light intensity on hydrogen isotope fractionation of long-chain alkenones produced by <i>Emiliania huxleyi</i>

“…Häggi et al (2015) did not find a significant relationship between δD C 37 values and salinity in suspended particulate matter from the Amazon Plume. In the Chesapeake Bay estuary (USA), δD C 37 values in sediments relate to δD C 37 values from suspended particulate matter filters and δD H 2 O values, but fractionation does not show a relationship with salinity (Schwab and Sachs, 2011).…”

Geochemistry of the Black Sea during the last 15 kyr: A protracted evolution of its hydrography and ecology

“…The salinity then increased to 24 S A during the dry Subboreal climate phase, to the onset of the Subatlantic climate phase at ~2.5 ka, when it gradually decreased to its current value of 18 S A . This model has its own uncertainties [ Chivall et al ., ; Anhäuser et al ., ; Haggi et al ., ], but they are reduced by analyzing the combined C 37 of alkenones [ van der Meer et al ., ]. I proposed above that the salinity of the photic zone increased during the less than 50 year period of the lacustrine/marine transition at ~7.8 ka (equation ; Figures and ), from 12 S A to some maximum value between 22 and 35.6 S A .…”

Salinity and stable oxygen isotope relationship in the Southwestern Atlantic: constraints to paleoclimate reconstructions

“…the isotopic fractionation of both oxygen and deuterium in seawater (δ 18 O sw and δD sw ) is mainly controlled by cofactors, such as temperature and salinity. the use of these stable isotopes together with thermohaline data have been widely applied in paleoceanographic reconstructions based on different matrices (Holloway et al 2015, Merlivat and Jouzel 1979, Minoura et al 1997, thornalley et al 2010, toledo et al 2007 in order to better understand the transition between glacial and interglacial periods. Another important application of δ 18 O sw and δD sw is in oceanographic circulation studies (Benway and Mix 2004, Conroy et al 2014, Craig and Gordon 1965, Kroopnick 1980, 1985, Pierre 1999, Pierre et al 1991, Venancio et al 2014.…”

“…For paleotemperature reconstructions, there are some methodologies and proxies that are already established, such as microfossil assemblage-based transfer functions and Mg/Ca ratios from foraminifera tests or Sr/Ca for coral (Elderfield and Ganssen 2000, Nurhati et al 2011, rohling 2007. however, proxies for salinity are still in development, especially to reduce the sources of uncertainty (which can be up to 4 psu) due to changes in regional freshwater budgets, ocean circulation and sea ice regimes (holloway et al 2015(holloway et al , Schmidt 1999.…”