A simple radiocarbon dating method for determining the age and growth rate of deep-sea sponges
Stewart Fallon1, Kelly James1, Rebecca Norman1, Michelle Kelly2 and Michael Ellwood1
1 Research School of Earth Sciences, Australian
National University, Canberra, ACT 0200, Australia
While radiocarbon-dating is a well established technique in aging marine carbonates, the ability to reliably age siliceous organisms remains largely unexplored. Attempts have been made to carbon date the proteinaceous material bound within fossil diatoms frustrules isolated from sediment cores [1,2] and siliceous sponges . The proteinaceous material bound within siliceous spicules is potentially a new and exciting way to age sponges and to date sediments devoid of carbonate material. Indeed the carbon dating of siliceous spicules in the Southern Ocean where carbonates are absent, or not well preserved, could help to validate paleoceanographic and paleoclimatic proxies and further elucidate physical and chemical changes within the oceans interior.
As filter-feeding organisms ubiquitous to the world’s oceans, marine sponges obtain carbon from the food they consume, incorporating low levels (0.05%) into the spicules they produce. The carbon intrinsically incorporated into the spicule matrix from the surrounding water, is protected from contamination and can potentially provide dates in opal rich sediment cores as well as providing information on extension rates in living siliceous organisms. The following is an interpretation of 14C analysis by accelerator mass spectrometry to constrain currently unknown growth rates of deep-sea sponges.
The Ross Sea sponge (TAN0402/67) was sub-sampled and cleaned using either sequential acid digestion alone, or acid digestion followed by roasting. Samples cleaned using sequential acid digestion alone needed to be placed inside a second quartz tubes to confer sufficient strength for combustion. In certain cases, these samples contained significantly more carbon than anticipated (Figure 1a). Elevated percentage carbon is attributed to insufficient removal of contaminant carbon (sponge tissue) by omitting the pre-roasting step.
The results for the percentage of carbon extracted and Δ14C for samples where pretreatment included incremental increases in roasting temperatures is presented in Figure 2b. These results indicate that the optimal roasting is >400 °C. Above this temperature all the external carbon is removed yielding low but consistent results for both percentage carbon recovered and the radiocarbon Δ14C results for proteinaceous material bound within the siliceous matrix (Figure 1b). The age results for sponge TAN0402/67 collected from the Ross Sea are presented in Figure 1c. A linear increase in age versus length was obtained for this sponge, although there is a significant water reservoir affect on the radiocarbon results. The estimate Δ14C value for organic carbon consumed by the sponge is about -150‰ based on modern organic carbon sedimentation in the Ross Sea. Our Δ14C results for the outer part of the sponge, at about -140‰, are close to the modern organic carbon sedimentation value thereby confirming that the sponge faithfully records the Δ14C signature of the organic carbon it consumes. After correcting for the reservoir age of the water, the extension rate for this sponge is around 2.9 mm yr-1. Using this extension rate and a length 15 cm along the axis of growth, we estimate that sponge TAN0402/67 is around 440 years old. This novel technique for elucidating extension rates in sponges and more broadly for dating siliceous organisms is testament to the broad applications of 14C/12C ratios by accelerator mass spectrometry as both a paleo- and modern oceanographic tool.
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