Comparison of climate signals obtained from encrusting and free-living rhodolith coralline algae

Abstract

Coralline algae have been used for sclerochronological studies throughout the last decade. These studies have focused on two different growth morphologies of the photosynthetic coralline algae: massive crusts forming small buildups on hard substrate, and free-living branching algal nodules, known as rhodoliths. The latter are generally found on soft-substrate, where they are frequently overturned by water movement and bottom feeding organisms, leaving one side of the rhodolith partially buried in the sediment at any given time. Here we test whether either of these growth morphologies is more suitable for proxy reconstructions by comparing Mg/Ca ratios-a temperature proxy-in multiple replicates of rhodoliths of Lithothamnion glaciale and in rhodoliths as well as encrusting specimens of Clathromorphum compactum. With both species being widespread throughout the Temperate and Arctic regions, we have chosen two North Atlantic localities at Nuuk Fjord, Greenland (Subarctic), and off the southeastern coast of Newfoundland, Canada (Temperate), for this study. Two to three Mg/Ca ratio transects spanning 18 years of growth were analysed on multiple specimens with encrusting morphologies and along different sides of rhodoliths using laser ablation inductively coupled mass spectrometry and compared to remotely sensed sea surface temperature (SST) data. The length of the common time span used for comparison was limited by growth interruptions in rhodoliths. Furthermore, our comparison is based on the assumption that rhodolith growth increments are annual-an assumption that has recently been challenged by mesocosm studies. Monthly Mg/Ca values from multiple transects within each individual were compared and in samples from Nuuk fjord significant correlations were found in 4 of 4 encrusting C. compactum, 4 of 4 C. compactum rhodoliths, and 2 of 3 L. glaciale rhodoliths. In Newfoundland significant correlations were found in 6 of 6 encrusting C. compactum comparisons (average: r=0.61, p<0.001), and in 6 of 6 L. glaciale rhodolith comparisons (average: r=0.43, p<0.001) for monthly resolved time series. The monthly Mg/Ca ratios (n=216) from each morphology were compared with instrumental Reynolds SST yielding the following correlations: encrusting C. compactum (r=0.64, p<0.001), C. compactum rhodolith (r=0.62, p<0.001) and L. glaciale (r=0.58, p<0.001). In Newfoundland both morphologies indicate a similar strength in recording SST: encrusting C. compactum (r=0.85, p<0.001) and rhodolith-forming L. glaciale (r=0.84, p<0.001). In summary, Mg/Ca ratios derived from both coralline algal growth forms can yield SST information, however, massive encrusting forms generally yield higher correlations to SST than transects measured on individual rhodoliths, which only allowed for the generation of short uninterrupted time series due to frequent growth irregularities. Highlights  Comparison of encrusting and rhodolith coralline algae for paleoclimate reconstruction  Both coralline algal forms can yield SST information, but encrusting forms...

Coralline Algae Archive Fjord Surface Water Temperatures in Southwest Greenland

“…The Mg/Ca ratio outliers shown in Figure are likely the result of a combination of biological variability and other environmental influences, such as light, on Clathromorphum Mg/Ca ratios (Moberly, ). Since specimen variability of Mg/Ca is common in Clathromorphum , it has been determined that averaging of multiple specimens reduces the impact of outliers on the time series (Hetzinger et al, ; B. Williams et al, ; S. Williams et al, ). This was taken into account in our longer time series by averaging five transects from two samples.…”

“…Since specimen variability of Mg/Ca is common in Clathromorphum, it has been determined that averaging of multiple specimens reduces the impact of outliers on the time series B. Williams et al, 2014;S. Williams et al, 2018).…”

Architecture and sequence stratigraphy of the Upper Coralline Limestone formation, Malta – implications for Eastern Mediterranean restriction prior to the Messinian Salinity Crisis

“…Prior studies (Bosence and Pedley, 1982;Pedley, 1979Pedley, , 1978 had identified the primary algae encruster as Lithophyllum and Mesophyllum, with encrusting foraminifera and bryozoans also present. The alternation of coralline bindstone and rhodoliths suggests variation in water agitation as rhodoliths are formed in an environment with more disruption (Williams et al, 2018). The high sphericity of the rhodoliths also points to a higher energy setting (Aguirre et al, 2017).…”

Growth Interruptions in Arctic Rhodoliths Correspond to Water Depth and Rhodolith Morphology

“…As a consequence of the short growing season at such high latitudes, the rhodolithforming specimens of L. glaciale grow very slow, with an average crust thickness increase of 0.075 ± 0.024 (SD) mm yr −1 (Teichert, own observations). Therefore, large L. glaciale rhodoliths with a crust thickness of several centimeters are usually several tens to hundreds of years old [1], which renders them as a potential archive for climate proxy data [6,7], as they often expose banding patterns with an annual resolution [8,9]. The annual banding Minerals 2021, 11, 538 2 of 13 pattern results from an alternation of larger cells formed in summer and smaller cells formed in winter [8], allowing for an assignation of each increment to a specific year, comparable to dendrochronology.…”

“…For this reason, when they appear in the shape of rhodoliths, the usability of coralline algae as geochemical archive is limited, because only skeletal transects without hiati can be used for such analyses. Consequently, when it comes to correlation measures between coralline algal geochemistry and environmental parameters like water temperature, coralline algal specimens that grow attached to a fixed substratum generally provide more reliable results than those that grow as unattached rhodoliths, since growth interruptions as a consequence of turning do not occur in attached specimens [6], as for example in the so-called clathrostromes built by Clathromorphum compactum (Kjellman) Foslie 1898 [16,17].…”