U-Th dating of deep-sea corals

Abstract

Abstract- 230 Th, 232 Th, 234 U and 238 U compositions of several deep-sea solitary corals, mainly the species Desmophyllum cristagalli, were determined by thermal ionization mass spectrometry (TIMS). It is possible to obtain high precision ages on modern pristine corals that have low [ 232 Th] (5 to a few hundred ppt). However, because older deep-sea corals tend to have higher [ 232 Th] compared to surface corals, and the initial 230 Th/ 232 Th ratio is uncertain, older deep-sea corals have larger age uncertainties (Ϯseveral hundred years for samples with a few thousand ppt 232 Th). Therefore, the key hurdle for precise U-Th dating is to remove or account for contaminants which contain elevated 232 Th and associated 230 Th not due to closed system decay within the coral lattice. A modification of the trace metal cleaning methods used for foraminifera and surface corals can significantly reduce this contamination. By counting the visible growth bands and measuring the mean age of a single septum, the extension rate of D. cristagalli was estimated to be between 0.1 and 3.1 mm/year. In a mean sense, bands appear to be precipitated annually, but this estimate has a large uncertainty. If appropriate tracer calibrations can be established, these corals are therefore suitable to record decadal or sub-decadal oceanographic changes over the course of their lifetime. The ␦ 234 U values of all modern samples from different localities and different depths are similar (mean 145.5 Ϯ 2.3‰) and indistinguishable from the data obtained from surface corals. At a precision of about Ϯ2‰, we find no structure in the oceanic profile of ␦ 234 U ratios over the top 2000 m of the water column.

Radiocarbon Dating of Deep-Sea Corals

“…Age "reversals" between the top and bottom of a single coral are therefore representative of a change in the seawater DIC at that site during the coral's lifetime. The 70 ± 28-yr age difference implies a mean growth rate of ~0.7 mm/yr, in good agreement with the U-series derived mean growth rates of several modern D. cristagalli (Cheng et al 2000). …”

“…Black, organic carbon rich crusts that form in-situ on fossil corals after death have been described in our earlier work (Cheng et al 2000). These crusts are diagenetic iron and manganese oxides that can trap significant amounts of detrital aluminosilicates.…”

230Th/238U dating of hydrothermal sulfides from Duanqiao hydrothermal field, Southwest Indian Ridge

“…The vent fluid and seawater from Lost City hydrothermal filed range from 92 (±116) to 183 (±46), 146.1 (±1.5) to 146.9 (±1.4), respectively (Ludwig et al 2011). The d 234 U initial of modern solitary deep sea corals ranges from 142.9 (±3.2) to 147.2 (±1.1) (Cheng et al 2000). The d 234 U initial of carbonate chimney samples from Lost City hydrothermal field range from 132.2 (±2.0) to 173 (±17) and the average chimney corrected initial d 234 U is 147.2 (±0.8) (Ludwig et al 2011).…”

“…A gravimetric U solution was also prepared by dissolving a weighted piece of NBS-CRM-112A metal, obtained from New Brunswick Laboratories. The 234 U/ 238 U ratio is (52.841 ± 0.082) 9 10 -6 , and d 234 U is (-38.7 ± 1.5) % (Cheng et al 2000Wang et al 2016). HU-1(Harwell Uraninite) and NBS Th were used as the standard for U and Th (Wang et al 2016).…”

The Holocene occurrence of cold water corals in the NE Atlantic: Implications for coral carbonate mound evolution

“…Deep-water corals can be accurately dated by means of U-series dating (Smith et al, 1997;Adkins et al, 1998;Mangini et al, 1998;Lomitschka and Mangini, 1999;Cheng et al, 2000a;Schröder-Ritzrau et al, 2003;Frank et al, 2004;Frank et al, 2005;Schröder-Ritzrau et al, 2005), which provides a tool to determine the age-depth relationship of coral fragments.…”