Daily growth and tidal rhythms in Miocene and modern giant clams revealed via ultra-high resolution LA-ICPMS analysis — A novel methodological approach towards improved sclerochemistry
Introduction
Biogenic carbonates with recognizable growth bands, such as mollusk shells, coral skeletons or fish otoliths, are of special interest for (paleo)environmental research (e.g. Alibert and McCulloch, 1997, Epplé et al., 2006, Meibom et al., 2007, Mertz-Kraus et al., 2009, Patterson, 1999, Schöne et al., 2002, Schöne and Gillikin, 2013, Stott et al., 2010, Vanhove et al., 2011, Yan et al., 2014b). Annual to sub-daily growth bands preserved in the biomineralized hard tissues of organisms, represent an accretionary record of (past) environmental conditions and may serve as direct lifespan control. Owing to their extraordinary size of more than one meter, longevity of more than 100 years and rapid shell growth rates of several mm/year, giant clams (family Cardiidae, subfamily Tridacninae, genus Tridacna) are particularly attractive sclerochronological archives for the tropical and subtropical regions of the Indo Pacific and have been extensively studied in the past ~ 30 years (e.g. Aharon, 1983, Aharon, 1991, Batenburg et al., 2011, Elliot et al., 2009, Sano et al., 2012, Warter et al., 2015, Welsh et al., 2011, Yan et al., 2013, Yan et al., 2015). The occurrence of a well-visible seasonal banding pattern within the aragonitic shell of Tridacna spp. is well-known and facilitated numerous studies on seasonal environmental variability (e.g. Batenburg et al., 2011, Elliot et al., 2009, Pätzold et al., 1991, Warter et al., 2015, Yan et al., 2014a). Less-utilized is the fact that Tridacna spp. secrete their shells daily, recorded as microscopically visible daily growth increments in the shell structure (Aharon and Chappell, 1986, Hori et al., 2015, Sano et al., 2012, Warter et al., 2015, Watanabe and Oba, 1999). Thus, environmental changes – reflected as variations in the shell's geochemical inventory – are continuously recorded in chronological order and at extremely high, possibly even hourly, temporal resolution.
Spatially-resolved geochemical analysis of successively secreted carbonate, via laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS), secondary ion mass spectrometry ((nano)SIMS) or electron micro probe analyzer (EMPA), enables retrieval of the trace elemental and isotopic inventory in chronological order, resulting in so-called time-series records. To ensure the accuracy of such time-series analysis, it is crucial to reliably identify and characterize daily or annual growth bands within the carbonate structure. To translate the detected (trace) elemental ratio variability into a temporal context of days, months or years, the usage of a second independent geochemical proxy, e.g. δ18O, might be necessary.
Owing to its potential for rapid and accurate high-resolution in situ trace element analysis at relatively low costs and minimal sample preparation requirements, LA-ICPMS has become a routine analytical tool in a wide area of research applications. Apart from analysis by discrete adjacent spots, compositional trace elemental variability in biogenic carbonates can be assessed using two different methodological approaches: (1) Lateral profiling, i.e. laser ablation along a defined transect on the sample's surface, typically oriented perpendicular to the accretionary growth direction and (2) Depth profiling, which involves static layer by layer removal of material at low laser repetition rates (1–2 Hz) to establish a depth-composition relationship (e.g. Eggins et al., 2004, Evans et al., 2015, Griffiths et al., 2013). The choice of sampling approach strongly depends on the size/thickness of the biogenic carbonate sample which depends on the growth rate and the overall lifespan of the organism, and the desired spatial/temporal resolution as well as the desired profile length, which determines the covered ‘time window’. Traditionally, LA-ICPMS lateral profiling is performed to produce long-term, up to centennial, time-series records with relatively low spatial and thus low temporal resolution of weeks to years (e.g. McCulloch et al., 2003). Lateral profiling therefore is suited for thick/large biogenic samples on the mm-scale, which exhibit well-defined annual growth layers, such as specific mollusk shells and coral skeletons (e.g. Alibert and McCulloch, 1997, Batenburg et al., 2011, Elliot et al., 2009, Gagan et al., 2000). On the other hand, LA-ICPMS depth profiling is the better approach for investigating trace elemental variation at (sub-)μm resolution, because the limiting factor for resolution is depth ablated per individual laser pulse (typically ≤ 0.1–0.15 μm) and not the diameter of the laser spot (Griffiths et al., 2013, MacDonald et al., 2008, Woodhead et al., 2008). It has been shown that it is possible to resolve daily compositional variability in planktonic foraminifera and fish otoliths using LA-ICPMS depth profiling (Eggins et al., 2004, MacDonald et al., 2008). This indicates that LA-ICPMS compares well with other high resolution methods, such as (nano)SIMS, which has been successfully employed to resolve daily compositional variability in foraminifera tests (Vetter et al., 2013), fish otoliths (Weidel et al., 2007), mollusk shells (Hori et al., 2015, Sano et al., 2012) and coral skeletons (Meibom et al., 2007). While LA-ICPMS depth profiling in principle allows highly spatially-resolved sampling, the depth range during static spot analysis is limited by the laser focal depth and the aspect ratio (depth/diameter) of the laser-drilled hole that in turn is controlled by its tapering angle. The achieved resolution via lateral profiling is determined by the utilized spot size. To maximize resolution along growth increments a rectangular spot should be used which facilitates substantially better spatial and thus temporal resolution compared to a round spot, while maintaining similar signal sensitivities (see details in Section 3.2).
Our primary aims are: (1) Introducing our methodology for ultra-high resolution LA-ICPMS analysis to the sclerochronology community to highlight that LA-ICPMS as the more rapid and inexpensive method compared to nanoSIMS is well-capable of resolving < 10 μm compositional cycles such as the growth increments in the aragonitic structure of giant clams. (2) Confirming that the presented μm scale (trace) elemental cyclicity can be characterized as daily, for which we utilize a Miocene specimen that has been extensively characterized in an earlier study (Warter et al., 2015). (3) Demonstrating that the chemical composition of the shell varies with the microscopically visible (micro)growth pattern. To this end we present complementary image processing results that link light intensity with measured trace elemental ratios. (4) Showcasing how long-term (annual) ultra-high resolution LA-ICPMS analysis reveals periodicities and their respective frequencies in giant clam shells.
Section snippets
Fossil Tridacna spp.
The presented Miocene Tridacna shells, LGS1 and BW4B, were sampled from the eastern coast of Borneo in the Indonesian province of East Kalimantan (0° 11′ 11.40″ N, 117° 26′ 40.92″ E; 0° 11′ 7.08″N, 117° 26′ 47.04″E), close to the city of Bontang (Fig. 1). The exteriors of both shells are altered, resulting in the loss of characteristic morphological features such as the fine details of the ribbing pattern and hinge area. Consequently, neither of the shells was identifiable to species level. A
Sample preparation
The Tridacna shells were thoroughly cleaned using deionized water and a tooth brush to remove organic matter and sediment particles from the external shell parts. The valves were then cross-sectioned along the maximum growth axis, from the umbo to the ventral margin, using a water-cooled Jencons Tyslide diamond saw, to expose internal growth increments. From opposite surfaces ~ 30 μm thick polished sections without cover (Fig. 2) were prepared and used for both microscopic examination of the
Results
The results of both LA-ICMPS analysis and image processing are displayed in Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8 and supplementary Fig. S1, Fig. S2, Fig. S3. The complete data set of all here presented LA-ICPMS profiles is provided as electronic supplementary material (Table S1–S6). Ablation of all transects was performed (unless otherwise specified) in direction of growth (‘dog’), i.e. towards more recently precipitated shell parts. All laser ablation transects were aligned perpendicular to
LA-ICPMS versus nanoSIMS for resolving daily trace elemental variability in giant clam shells
This study expands upon earlier nanoSIMS work by Sano et al. (2012) and Hori et al. (2015), who successfully extracted daily compositional variability in modern (Mg/Ca, Sr/Ca) and Holocene giant clams (Mg/Ca, Sr/Ca, Ba/Ca) via discrete spot analysis at 2 μm resolution.
Here, we utilize a different methodological approach – ultra-high resolution LA-ICPMS – and extend the (trace) elemental suite to also include B and Y, besides Mg, Sr, Ba and Ca. By utilizing a rotating rectangular laser spot (4 × 50
Conclusions
Our novel approach for ultra-high resolution LA-ICPMS analysis enables resolution of < 10 μm compositional variability in B/Ca, Mg/Ca, Sr/Ca and Ba/Ca preserved within microscopically visible daily growth increments in the aragonitic structure of modern and Miocene giant clams. LA-ICPMS allows significantly faster and less cost intensive analysis compared to nanoSIMS and compares well with the results by Sano et al. (2012) and Hori et al. (2015), who resolved daily compositional trace elemental
Acknowledgments
A special thanks to all people and authorities involved in the sampling of the modern Tridacna squamosa shell, namely Aaron Hunter (Curtin University, Perth) and Jasmin Saw (SEAaRL - Universiti Teknologi PETRONAS, Malaysia) for collecting and exporting the specimen (funded by URIF, UTP, CITES Permit Certificate DOF(S)1611), Datuk Ali Lamri and Maklarin Lakim at Sabah Parks for permission to work on Pulau Sibuan, Tun Sakaran Marin Park, and Willem Renema (Naturalis Biodiversity Center, Leiden,
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