Alkenone Distributions and Hydrogen Isotope Ratios Show Changes in Haptophyte Species and Source Water in the Holocene Baltic Sea

Gabriella M Weiss,Marcel T J Van Der Meer,Stefan Schouten,Barbara Massalska,Gert‐Jan Reichart,Rick Hennekam,Jaap S Sinninghe Damsté

doi:10.1029/2019gc008751

Gabriella M Weiss, Marcel T J Van Der Meer + Show 5 more

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https://doi.org/10.1029/2019gc008751

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Abstract

AbstractThe Baltic Sea, a dynamic, marginal marine basin, experienced a number of large changes in salinity during the Holocene as a result of fluctuations in global and local sea level related to melting of glacial ice sheets and subsequent isostatic rebound. These changes likely had pronounced effects on the species composition of haptophytes, a common phytoplankton group found in the Baltic Sea. This dynamic environment provides the ideal setting to study how species change impacts distribution and hydrogen isotope ratios of long‐chain alkenones (δ2HC37), haptophyte‐specific biomarkers. Here we analyzed the aforementioned parameters in Holocene sediments covering the contrasting hydrological phases of the Baltic Sea. Alkenone distributions changed with different Baltic Sea salinity phases, suggesting that species shifts coincide with salinity change. δ2HC37 values show two major shifts: one in the middle of the freshwater Ancylus Lake phase (10.6 to 7.7 ka) and a second at the transition from the brackish Littorina Sea phase (7.2 to 3 ka) into the fresher Modern Baltic (3 ka to the present). The first shift represents a significant enrichment of 50‰, which cannot be explained by salinity or species changes only. At this time, the isotopically depleted ice sheets had melted, and only the relatively enriched freshwater source remained. The second shift, coincident with a change in distribution, is likely caused by a change in species composition alone. These findings show that hydrogen isotope ratios of long‐chain alkenones, combined with their relative distribution, can be used to reconstruct changes in source water.

Highlights

Haptophyte algae are important primary producers and believed to be one of the most substantial producers of calcium carbonate globally (Laguna et al, 2001; Paasche, 2002) inhabiting both lacustrine and marine environments (Theroux et al, 2010)
Alkenone distributions changed with different Baltic Sea salinity phases, suggesting that species shifts coincide with salinity change. δ2HC37 values show two major shifts: one in the middle of the freshwater Ancylus Lake phase (10.6 to 7.7 ka) and a second at the transition from the brackish Littorina Sea phase (7.2 to 3 ka) into the fresher Modern Baltic (3 ka to the present)
The second shift, coincident with a change in distribution, is likely caused by a change in species composition alone. These findings show that hydrogen isotope ratios of long‐chain alkenones, combined with their relative distribution, can be used to reconstruct changes in source water

Summary

Introduction

Haptophyte algae are important primary producers and believed to be one of the most substantial producers of calcium carbonate globally (Laguna et al, 2001; Paasche, 2002) inhabiting both lacustrine and marine environments (Theroux et al, 2010). The hydrogen isotope composition of long‐chain alkenones (δ2HC37) has been shown to strongly relate to salinity and the δ2H values of water (e.g., Chivall et al, 2014a; M'Boule et al, 2014; Sachs et al, 2016; Schouten et al, 2006; Weiss et al, 2017), which has led to the use of δ2HC37 values to reconstruct paleosalinity in a number of different marine environments (Pahnke et al, 2007; van der Meer et al, 2007, 2008; Leduc et al, 2013; Kasper et al, 2014, 2015; Petrick et al, 2015; Simon et al, 2015; Weiss, de Bar, et al, 2019) Other factors such as growth rate (Sachs & Kawka, 2015), growth phase (Chivall et al, 2014a; Wolhowe et al, 2009), and light intensity (van der Meer et al, 2015; Weiss et al, 2017) are known to affect the δ2H values of alkenones. While the latter factors exhibit significant controls on δ2H values in culture, environmental studies do not always reflect these effects and often salinity variations appear to be the strongest signals (Weiss, Schouten, Sinninghe Damsté, & van der Meer, 2019, and references therein)

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