News

Mg/Ca ratios are regularly tested to see whether they can be used for reconstructing water temperatures, but are also often shown to be not very reliable. So mapping the Mg/Ca ratios using LIBS would give us pretty a good idea of what's going on in this case. Our new setup uses a combination of cleaning shots and measurement shots. Cleaning shots that take off ~0.1�g from the surface and expose fresh carbonate, while measurement shots actually measure Magnesium and Calcium concentrations, using our trusted peaks of Mg and calcium ions with their respective wavelengths of 279 nm and Ca ions at 315 nm. The intensity of both peaks is based on the intensity of emitted photons with the above wavelengths. With each shot we create a plasma that emits photons based on the plasma's elemental composition. That way we can get a linear correlation between the peaks' intensities and the actual Mg/Ca ratio within the material.

Looking at the shells in general, there are two layers of the shell that grow pretty much in sync. One is made of calcite (coloured) and one is made of aragonite (black). The reason why the aragonitic (or inner) layer is black, is because it contains a much lower ratio of Mg to Ca and can pretty easily be identified this way.

The calcitic (or outer) layer of the shell has Mg/Ca ratios that range between 0.05 and 0.25.

Another thing that became clear is that all shells are different. These differences range from trivial things, like the holes from Christopher's earlier drilling for carbonate samples in January's shell #1, to less trivial things like growth interruptions and mixing of calcite and aragonite layers in May's shells #2 and #3.

The inconsistent way that these layers are mixed, makes them very difficult to analyse and to track changes in element concentration through time (so I didn't )

The Mg/Ca ratio is mostly consistent within growth increments, which suggests that whatever controls the ratio is doing this at the point in time that the growth specific increments represent. This is different from the high and low ratios we find in the calcite and aragonite parts which grew at the same time, because their respective mineralogies are already biased towards one way or the other.

While all shells show some kind of banding patterns with high and low Mg/Ca ratios, these bands are not consistently distinct in all specimens. For comparison, there is map of a shell further below, with very distinct patterns from Christopher's March collection.

How come this distinctiveness is not always the case? - I don't know.

Could this be age related, since bands are getting gradually more distinct in this shell? - We'll need more older shells to determine this. What could the Mg/Ca increases actually mean? - Tentatively, it could be argued that they are seasonal patterns that happen every year and high Mg/Ca ratios in one shell relate to similar times in another shell (stroked lines illustrate this). Sadly the inconsistency of the patterns, especially in shells from May, makes this a somewhat useless bit of information, which can't be relied on.

More importantly, the concept that Mg/Ca is mostly temperature related is seemingly not the case in our samples, because a) the factors behind the distortion of patterns are clearly in control of Mg/Ca ratios and dominate the record and b) the Mg/Ca ratios measured at the shell edges of January shells have a higher Mg/Ca ratio than the May shells, where water temperature (and thus Mg/Ca ratios) should be warmer.

This preliminary data leaves us with more questions to solve next:

• Are the distinctly high-ratio bands related to seasonal change?
• What causes the higher amounts of Mg/Ca in those bands (seasonal or structural factors)?
• Can the banding patterns be used to guide sampling for stable isotope analysis?
• What causes irregularities or the lack of them?
• Can these causes be anticipated by distinct exterior features of the shell?
• Are irregularities more prominent in younger shells/younger parts of old shells?

To answer this, we'll need many more shells to find common features and additional contextual information to characterise them.

Until soon,

Niklas

Posted on November 2, 2017 byNiklas Hausmann

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Our LIBS setup now uses a new focusing lens for infrared light and with 10x magnification, which decreases our sampling area to a diameter of ~40 �m. We also added a cleaning step before measuring each location, by simply using a number of shots that ablate the material at the surface and expose "fresh" carbonate. The sampling rate has been improved somewhat, by simply taking less spectra per location. The peaks for Mg at 279.6 nm and for Ca at 315.9 nm are actually clear enough to map shells with a single spectrum instead of our usual 10 spectra per location. That way we can get almost 4,500 sample points per hour (0.81s/point). Previously this was 2,100 sample points per hour. Sadly, the Sr peak at 407.8 nm is less clear and single spectra are too variable for my taste, which is why 3 spectra per sample point seemed more appropriate, when measuring Sr/Ca. Sampling rate here is about 3,600 per hour (1s/spot).

These numbers can still be improved by using newer equipment (ours is on average 15 years old). Right now the biggest problem is our pretty old computer and the fact, that the sample stages do not move constantly, but stop at each point to take one or more spectra. Single spectra analysis of Mg/Ca could allow us to have the stages move at a constant speed and make full use of our 10 Hz laser. That way we could achieve an almost tenfold increase in speed (not counting the cleaning step). So that is something to aim for before the end of the project in 10 months(!).

I was hoping to update this project more frequently, but in the next few weeks, I'll be busy with collecting more samples in Denmark as well as with Matthew Meredith-Williams in Melbourne. But I am looking forward to sharing new data as soon as it becomes available. Until then!

Posted on July 24, 2017 by Niklas Hausmann

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Submerged landscapes play a key role in human history, with long periods of the last 200 thousand years having experienced lower sea levels than we know today.

In relation to this, not all but many of the currently submerged underwater archaeological sites contain traces of coastal exploitation and shellfish remains, which is why the ACCELERATE Project addresses a major source of archaeological and ecological information. Preliminary data on LIBS, shell carbonates and environmental change can be found in the new poster, which can be downloaded here.

Posted on July 24, 2017 by Niklas Hausmann

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To test the performance of my experimental setup, I analysed three shells of marine and lagunar mollusc species, including one gastropod and two bivalves: a fragment of the aperture lip of Conomurex fasciatus (Born 1778), a hinge of Ostrea edulis (Linnaeus 1758), and one complete half of Anomalocardia flexuosa (Linnaeus 1767) from a modern context. The shells are associated with three different sites, from different periods and different regions of the world. The aim here was not to compare three records of the same environment, but to test the experimental setup on a variety of species

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Method

LIBS is a simple, straightforward and versatile technique for rapid analysis of the chemical composition of a huge variety of materials (Harmon et al., 2013). It is based on the atomic emission spectroscopy of plasma, generated by focusing a high intensity laser beam onto the material. The plasma emission is collected and then spectrally resolved. This provides qualitative and quantitative results indicated by the emission line intensities of the elemental constituents of the material.

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Fig. 1. Experimental setup using an acquisition process over two phases. Red arrows indicate the process of sample targeting, blue arrows indicate the process of irradiation and measurement. During acquisition, an infrared laser beam is focused on the sample to create plasma in a ~90�m sample area. The emitted light is collected, spectrally resolved and instantly displayed as spectrum on the computer.

In the examples here, I chose to measure the intensities of emission lines which correspond to ions of the elements Mg and Ca. The ratios calculated reflect the intensities of specific emission lines and do not directly translate into ratios based on weight or molar ratios, although the relationship is somewhat linear. Hence the ?intensity-ratios? here are reported in arbitrary units (a.u.).

The setup?s acquisition process was programmed using LabView software (www.ni.com/labview/) and is divided into two phases (indicated by red and blue arrows in Fig. 1). In the first phase (red), multiple target locations or waypoints are being selected by moving the sample stage to the preferred location, which is visible through the camera. The sample locations are then interpolated between the two (or more) waypoints at a given interval (here 100�m) and put down in a list.

In the second phase, the motorised stage goes through the list of pre-selected sample locations, while acquiring LIBS spectra at each location with an irradiation area of ~90�m in diameter. In each spectrum the mean ratio of the intensity of the two pre selected peaks is calculated and then assigned to the corresponding sample location in the list.

There are two modes that can be chosen from for the second phase of acquisition. The first mode (accumulation) calculates the ratio of selected peaks in a single ?accumulated? spectrum which is an accumulation of several spectra (here 10) in one location. The second mode (single shot) calculates the ratio of selected peaks in each of the 10 spectra separately. While the resulting mean ratio is identical, the two modes have different advantages. The accumulation mode has a faster performance because only one spectrum needs to be analysed, whereas the single shot mode allows the user to assess the standard deviation of the 10 spectra to provide an error for the mean value. As such, the accumulation mode is more suitable for spatially mapping elemental ratios and the single shot mode is more suitable for tracking changes along the direction of growth and thus through time. The typical time spend in one sample location is about 1.7 seconds in accumulation mode and 2.4 seconds in single shot mode. With this configuration a typical analysis of 5 cm translates into 500 sample locations (5,000 spectra) and would required a time of acquisition of about 14 minutes (20 min in single shot mode).

Experiments

For the experiment, I used three samples. In all three cases the shell had previously been sectioned along the direction of growth and thus provided a flat surface for me to analyse.

The sections were analysed in both modes (Fig. 2). First, I created a map of the elemental ratios by using the accumulation mode. This allowed me to get a good idea of larger trends in the elemental composition of the shell and to identify patterns that are likely not related to environmental change. Only afterwards I selected an area of interest (black lines) that was analysed in the more detailed single shot mode following the direction of growth.

All three shells produced Mg/Ca intensity ratios that described somewhat cyclical patterns, as is expected when analysing records on a seasonal scale. However, all specimens also had isochronous parts within the same growth increments where the values would deviate from the general cyclical trend, indicating that other factors control the elemental ratio as well (Marali et al., in press).

Fig. 2. LIBS results of three shell samples. Left column: mapped Mg/Ca intensity ratios of shell samples with direction of growth and sample path for single shot mode indicated (black dashed line), colour-mapping identical to graph on the respective right with red being high values and blue being low values. Right column: Mg/Ca intensity ratios plotted by distance to edge, colours of data points identical to mapped values.

In C. fasciatus, the single shot mode shows high values at a 0?2 mm distance to the shell edge, with a short decrease at 0.3 mm, before a larger decrease from 0.09 Mg/Ca to 0.04 Mg/Ca at 3.0 mm. Afterwards, there is an increase to above 0.10 Mg/Ca over the next 4 mm. Despite this being the general trend within the sample, the map of ratios shows quite clearly that some parts outside the sampling path of the linear measurement (black line) deviate from those values. Similar spatial trends were found by Poulain et al. (2015) in Ruditapes philippinarum and indicate a non-environmental influence.

The O. edulis sample shows similar results. Measuring along the direction of growth in single shot mode, a repetitive pattern is apparent with peaks at at ~0.17 Mg/Ca and lows at ~0.11 Mg/Ca. However, the map shows areas in the top right that generally produce higher values than areas in the bottom left, even though the areas cover increments that grew at the same time. This could be explained with the averaging of growth increments in the bottom left of the map, that are too thin to be sampled individually. Potentially high values could be mixed together with other thin growth increments that produce lower values (Shirai et al., 2014). However, this effect would also prevent extremely low values, which were still reached.

Variations of Mg/Ca values outside the linear sampling path were also found in A. flexuosa. The mapped ratios separate the shell section into three parallel growing layers (interior, centre, and exterior). Albeit growing at the same time, they almost consistently produce different Mg/Ca values, with the central one having higher values than the others. Similar ?zonation? was found by Lazareth et al. (2013) in Protothaca thaca and Freitas et al. (2009) in Mytilus edulis as well as Pecten maximus, who suggest sampling P. maximus within the mid-regions of the shells for more consistent environmental proxy data.

The reason behind the spatial patterns in isochronous parts of the shells analysed above requires more thorough analyses, before they can be used as seasonal or environmental proxy. Although there are cyclical variations that could well be controlled directly or indirectly by seasonal environmental change, it will be necessary to clearly link element/Ca ratios to specific changes. Otherwise it will be difficult to apply the same seasonal control to archaeological specimens and make consistent interpretations of the archaeological dataset.

Summary

With these experiments I showed the application of an automated LIBS setup to acquire elemental ratios from in situ shell carbonate. This was realised with the use of a LIBS imaging setup that offered a spatial resolution at a scale of ~100 �m and an acquisition speed of ~20,000 spectra/h. The setup proved the feasibility of efficiently acquiring elemental ratios from shell carbonates using LIBS, which will directly translate into lower costs per data point.

However, I have not yet linked the elemental ratios of the LIBS values to distinct environmental change, although cyclical changes were visible that suggest seasonal patterning. A validation based on modern shells and contemporaneous environmental data is still needed to fully make a case for the use of the presented elemental ratios as seasonal proxies.

This will be the next step of the ACCELERATE project. The variability of the elemental ratios in isochronous parts will make this more difficult and it is likely that different solutions need to be found for different shell species and environments. That said, the possibility to quickly assess elemental ratios will be beneficial in carrying out the analyses of more shell specimens in a shorter time frame than previously possible.

References

Freitas, P.S., Clarke, L.J., Kennedy, H., Richardson, C.A., 2009. Ion microprobe assessment of the heterogeneity of Mg/Ca, Sr/Ca and Mn/Ca ratios in Pecten maximus and Mytilus edulis (bivalvia) shell calcite precipitated at constant temperature. Biogeosciences 6, 1209?1227.

Garc�a-Esc�rzaga, A., Moncayo, S., Guti�rrez-Zugasti, I., Gonz�lez-Morales, M.R., Mart�n-Chivelet, J., C�ceres, J.O., 2015. Mg/Ca ratios measured by laser induced breakdown spectroscopy (LIBS): a new approach to decipher environmental conditions. J. Anal. At. Spectrom. 30, 1913?1919.

Graniero, L.E., Surge, D., Gillikin, D.P., Briz i Godino, I., �lvarez, M., 2017. Assessing elemental ratios as a paleotemperature proxy in the calcite shells of patelloid limpets. Palaeogeogr. Palaeoclimatol. Palaeoecol. 465, Part B, 376?385.

Harmon, R.S., Russo, R.E., Hark, R.R., 2013. Applications of laser-induced breakdown spectroscopy for geochemical and environmental analysis: A comprehensive review. Spectrochim. Acta Part B At. Spectrosc. 87, 11?26.

Hausmann, N., Meredith-Williams, M., 2016. Exploring Accumulation Rates of Shell Deposits Through Seasonality Data. J Archaeol Method Theory. doi:10.1007/s10816-016-9287-x

Jew, N.P., Erlandson, J.M., Rick, T.C., Reeder-Myers, L., 2013a. Oxygen isotope analysis of California mussel shells: seasonality and human sedentism at an 8,200-year-old shell midden on Santa Rosa Island, California. Archaeol. Anthropol. Sci. 6, 3, 293?303 doi:10.1007/s12520-013-0156-1

Jew, N.P., Erlandson, J.M., Watts, J., White, F.J., 2013b. Shellfish, Seasonality, and Stable Isotope Sampling: δ¹⁸O Analysis of Mussel Shells From an 8,800-Year-Old Shell Midden on California?s Channel Islands. The Journal of Island and Coastal Archaeology 8, 170?189.

Lazareth, C.E., Le Cornec, F., Candaudap, F., Freydier, R., 2013. Trace element heterogeneity along isochronous growth layers in bivalve shell: Consequences for environmental reconstruction. Palaeogeogr. Palaeoclimatol. Palaeoecol. 373, 39?49.

Marali, S., Sch�ne, B.R., Mertz-Kraus, R., Griffin, S.M., Wanamaker, A.D., Butler, P.G., Holland, H.A., Jochum, K.P., in press. Reproducibility of trace element time-series (Na/Ca, Mg/Ca, Mn/Ca, Sr/Ca, and Ba/Ca) within and between specimens of the bivalve Arctica islandica--A LA-ICP-MS line scan study. Palaeogeogr. Palaeoclimatol. Palaeoecol. doi:10.1016/j.palaeo.2016.11.024

Poulain, C., Gillikin, D.P., Th�bault, J., Munaron, J.M., Bohn, M., Robert, R., Paulet, Y.-M., Lorrain, A., 2015. An evaluation of Mg/Ca, Sr/Ca, and Ba/Ca ratios as environmental proxies in aragonite bivalve shells. Chem. Geol. 396, 42?50.

Shirai, K., Sch�ne, B.R., Miyaji, T., Radermacher, P., Krause, R.A., Jr., Tanabe, K., 2014. Assessment of the mechanism of elemental incorporation into bivalve shells (Arctica islandica) based on elemental distribution at the microstructural scale. Geochim. Cosmochim. Acta 126, 307?320.

Thomas, K.D., 2015. Molluscs emergent, part II: themes and trends in the scientific investigation of molluscs and their shells as past human resources. J. Archaeol. Sci. 56, 159?167.

Thompson, V., Andrus, C., 2011. Evaluating Mobility, Monumentality, and Feasting at the Sapelo Island Shell Ring Complex. Am. Antiq. 76, 315?344.