Pliocene diatom and sponge spicule oxygen isotope ratios from the Bering Sea: isotopic offsets and future directions

Abstract

Abstract. Oxygen isotope analyses of different size fractions of Pliocene diatoms (δ 18 O diatom ) from the Bering Sea show no evidence of an isotope offset and support the use of bulk diatom species samples for palaeoceanographic reconstructions. Additional samples containing concentrations of sponge spicules produce δ 18 O values several per mille (‰) lower than δ 18 O diatom with a calculated mean offset of 3.9‰ ± 1.5. This difference is significantly greater than modern-day variations in water δ 18 O through the regional water column. Despite the potential for oxygen isotope disequilibrium within δ 18 O sponge , there appears to be some similarity between δ 18 O sponge and a global stacked benthic δ 18 O foram record. This highlights the potential for δ 18 O sponge in palaeoenvironmental research at sites where carbonates are not readily preserved.

Technical Note: Silica stable isotopes and silicification in a carnivorous sponge <i>Asbestopluma</i> sp.

“…The fractionation factor ( δ 18 O silica−seawater ) for the marine sponge (+36 to +39 ‰) was greater than that of freshwater sponges (+28 ‰) (Matteuzzo et al, 2013). The variation within the one individual from this study compares to an entire range of δ 18 O water of less than 0.8 ‰ and potential temperature variations of ∼ 5 • C across the Drake Passage (Meredith et al, 1999), and represents nearly half of the 5 ‰ variations found in a downcore sponge spicule δ 18 O record from Pliocene sediments (Snelling et al, 2014). The external spicules were consistently isotopically lighter than the internal interlocking spicules, although the difference between them (0.4 to 1 ‰) is approximately the same as the analytical error (2SD of 0.6 ‰).…”

“…Heating to higher temperatures of 80-90 • C is routine in downcore spicule opal δ 30 Si analyses (e.g. Snelling et al, 2014). …”

“…Ellwood et al, 2010;, there is still scatter in the calibration of the δ 30 Si-Si(OH) 4 relationship, with the sources of variability poorly understood. Likewise, little is known about the sponge spicule silica δ 18 O, although it is likely impacted by biological factors (Matteuzzo et al, 2013) that cause systematic offsets when compared to diatom silica δ 18 O (Snelling et al, 2014). Here, we investigate the impact of derived biomineralisation mechanisms that could be responsible for variations in silicon and oxygen isotope fractionation in sponges using a carnivorous sponge specimen from the Southern Ocean as a case study.…”

Technical Note: Silica stable isotopes and silicification in a carnivorous sponge \\textit{Asbestopluma} sp.

“…The formation of amorphous biogenic silica (or opal) by photosynthetic diatoms, which play a major role in the export of organic matter to the seafloor, is a key part to both the cycling of silicon and carbon in the Earth's climate system (Tréguer and De la Rocha, 2013). Quantifying the dissolved silicon, or silicic acid (Si(OH) 4 ), concentration of upwelling waters is essential if we are to understand the distribution and growth of diatoms in surface waters and so the drawdown on atmospheric carbon dioxide (Hendry and Brzezinski, 2014).…”

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…A study of a marine carnivorous sponge δ 18 O showed large fractionation effects and highly heterogeneous isotopic behavior within one individual (Hendry et al, 2015). One downcore study indicates that there are systematic offsets between spicule and diatom bSiO 2 δ 18 O (Snelling et al, 2014).…”