Intra‐annual variation in the stable oxygen and carbon and trace element composition of sclerosponges

Abstract

[1] This paper presents data to support the presence of (1) intra-annual signals in the chemical composition (d 18 O and Sr/Ca) of the skeletons of sclerosponges from the Bahamas and (2) variable rates of skeletal accretion. These conclusions are based on data obtained by using a microsampling method for the stable oxygen and carbon isotopes in which material was extracted at a resolution of one sample every 34 mm and a laser microprobe which obtained trace element data every 20 mm (Sr, Mg, and Pb). An age model was established using a combination of changes in the concentration of Pb, the change in the d 13 C of the skeleton of the sclerosponges, and U/Th isotopic measurements. These methods yield a mean growth rate of 220 mm/yr but suggest that the growth rate in this particular sclerosponge was not constant. The calculated growth rate is within error identical to that determined by U/Th methods. The variable growth rate was confirmed through spectral analysis of the d 18 O and Sr/Ca data that showed peaks corresponding to the annual cycle in these parameters as well as peaks corresponding to growth rates of approximately 128, 212, 270, and 400 mm/yr. The presence of these additional frequencies suggests a growth rate between approximately 100 and 300 mm/yr. These conclusions were supported by modeling of oxygen isotopic data measured on a scleractinian coral as well as model isotope data generated on synthetic time series. These findings have important implications for the use of sclerosponges as proxies of paleoclimate because they emphasize the need for a precise yearly chronology in order that proxy data can be compared with climatic variables.

Extracting growth rates from the nonlaminated coralline sponge Astrosclera willeyana using bomb radiocarbon

“…These data show that sponge growth rate may vary by a factor of two, indicating that single linear extension growth rates are not valid. This type of growth rate variation has been observed in other coralline sponges such as Ceratoporella nicholsoni (Swart et al 2002;Willenz and Hartman 1999). Some of the growth rate variability within an individual may be due to errors associated with matching the two ∆ 14 C curves or to the time error associated with the original coral ∆ 14 C records.…”

Interpreting environmental signals from the coralline sponge Astrosclera willeyana

“…This is accomplished by measuring isotopic and elemental ratios incorporated into the coral skeleton while it grows. To explore thermocline and deeper water variability, techniques historically applied to reef building corals are being applied to coralline sponges (Fallon et al, 2003a;Fallon et al, 1999a;Haase-Schramm et al, 2003;Lazareth et al, 2000;Rosenheim et al, 2004;Swart et al, 2002).…”

Constraining initial 230Th activity in incrementally deposited, biogenic aragonite from the Bahamas

“…An assumption of zero [ 230 Th/ 232 Th] i has been made in several projects involving the U/Th dating of young marine carbonates (Barnes et al, 1956;Edwards et al, 1986;Bö hm et al, 1996Bö hm et al, , 2000Moore et al, 2000;Swart et al, 2002;Haase-Schramm et al, 2003). In such cases, when initial activities of thorium are assumed negligible, the 230 where d 234 U m is the oceanic secular disequilibrium which is measured for each sample, but in the vicinity of 146 ± 10‰ (Bard et al, 1991;Stirling et al, 1998;Cheng et al, 2000b;Henderson, 2002;Robinson et al, 2006).…”