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doi:10.2204/iodp.proc.325.102.2011

Chronology

The aim of preliminary dating was to provide a rapid overview of the ages of the material recovered during the offshore phase of Expedition 325. These dates are intended to aid the onshore phase of the expedition and, in particular, the sampling of the core material so that more focused sampling strategies can be implemented to achieve the scientific objectives of individual science party members. We hope that this dataset will act as a starting point for collaborations within the science party. The data will also enable more effective outreach to the scientific community in the period immediately following the science party before more comprehensive chronologies become available.

Sampling

Subsamples of core catcher material were taken during the offshore phase under the direction of the Co-Chief Scientists. Coral and mollusk material was selected based on the likelihood of the calcium carbonate being suitable for U-Th or radiocarbon dating (no visible diagenetic textures and free of detrital contamination) and from an important core section so as to constrain the basic chronology of each hole. A total of 20 coral samples were sent to the University of Oxford (United Kingdom) for U-Th dating, and 48 samples were sent to the University of Tokyo (Japan) for sample preparation and then the Australian National University (Australia) for radiocarbon analysis. To ensure that the chronological control on the lower portions of holes was not limited by the range of the radiocarbon chronometer (~50 cal y BP), the deeper samples from each hole were selected for U-Th analysis and the shallower samples were analyzed for radiocarbon.

U-Th measurements

Samples were reduced to pieces ~5 mm in size, ultrasonically cleaned in Milli-Q (18 MΩcm) water, and examined for detrital contamination. Approximately 0.3 g of the cleanest coral aragonite was picked for U-Th analysis, and a similar amount was set aside for radiocarbon analysis. Sample preparation was the only screening performed on the subsamples, in contrast to the normal approach to U-Th dating where samples are rigorously screened for diagenetic alteration prior to analysis. This omission of sample screening greatly improved the timely throughput of the samples, allowing the data to be used to assess the ages of samples to the accuracy required to improve sampling strategies during the OSP.

Coral subsamples for U-Th analysis were weighed, spiked with a mixed ²²⁹Th:²³⁶U tracer (Robinson et al., 2004a), and dissolved in HNO₃. Once dissolved, samples were refluxed in a mixture of 3:1 HNO₃(conc):HCl(conc) (reverse aqua regia) overnight to remove organic matter and ensure sample tracer equilibrium. Samples were converted to nitrate form, followed by separation of U and Th from each other and the sample matrix (Negre et al., 2009).

U and Th isotopes were measured using a Nu Instruments multicollector inductively coupled plasma–mass spectrometer (MC-ICP-MS) at the University of Oxford. U was measured statically with ²³⁴U in an ion counter, and all other isotopes were counted in faraday collectors. All machine biases were accounted for by bracketing each sample measurement with measurements of the NBS 112a uranium standard, with the mass 234 beam intensities matched between samples and standards. Th was measured dynamically, with both ²³⁰Th and ²²⁹Th measured sequentially in an ion counter, and differences in beam intensity between measurement steps were normalized through measurement of ²³²Th in faraday collectors during each step. Again, machine biases were corrected using a bracketing standard and, in this case, an internal ²²⁹Th:²³⁰Th:²³²Th (ThIS-2; Mason and Henderson, 2010) standard was used.

14C measurements

Coral and mollusk samples for radiocarbon measurements were further divided using a dental drill to obtain calcium carbonate with minimal diagenetic alteration. Subsamples (~50 mg) were ultrasonically cleaned in Milli-Q water and then dried in an oven, and ~40%–50% of the material was etched prior to conversion to CO₂. Neither stepwise dissolution (Yokoyama et al., 2000) nor X-ray diffraction measurements were conducted to screen for diagenesis in order to maintain the same constraints imposed as for U-series dating.

All cleaned calcium carbonate samples were placed in test tubes and dissolved in phosphoric acid. The evolved CO₂ was then cleaned in an ultra vacuum preparation line and reduced to graphite in a hydrogen atmosphere (Yokoyama et al., 2007, 2010). Radiocarbon in the graphite was then measured by accelerator mass spectrometry (AMS) (Fallon et al., 2010; Yokoyama et al., 2010).

Data

U and Th isotope ratios and age determinations are shown in Table T7, and uncalibrated radiocarbon ages are presented in Table T8.

Age interpretations

U-Th

Closed system ages

U-Th ages of samples can be calculated using the age equation of Broecker (1963) under the assumptions that the sample has remained a closed system to ²³⁸U, ²³⁰Th, and their intermediate nuclides, and that the sample had no initial ²³⁰Th. The results of these calculations, as well as the backcalculated (²³⁴U/²³⁸U) at the time the coral grew ([²³⁴U/²³⁸U]_i), are shown in Table T9.

Initial Th correction

The assumption that the coral samples contained no initial ²³⁰Th is not always valid. In some of the coral samples, ²³²Th concentrations are elevated relative to that expected for pristine coral aragonite (>0.1 ppb), suggesting that there has been some incorporation of Th. High ²³²Th concentrations (as much as 110 ppb in Sample 325-M0033A-15R-2, 8 cm) suggest that a significant continental detrital component is present (it is unlikely that ²³²Th incorporation from seawater alone could cause such an enrichment). The incorporation of ²³²Th into the coral when it formed would have been associated with the incorporation of some ²³⁰Th as well. This initial ²³⁰Th requires accounting for during interpretation of the age. If all of the initial Th was derived from continental detrital material, then one could use the crustal average ²³⁰Th/²³²Th value (1.030; median of GEOROC Database [georoc.mpch-mainz.gwdg.de/georoc/]) and use the ²³²Th concentration in the sample to assess the amount of initial ²³⁰Th. The effect of correcting for this initial ²³⁰Th using a crustal value is shown in Table T10 and Figure F11 (an uncertainty of 100% is associated with each estimate of initial ²³⁰Th to account for the uncertainty in the [²³⁰Th/²³²Th] of the contaminant). However, it is possible that samples with lower ²³²Th concentrations incorporated a significant portion of their Th from seawater, which has a much higher ²³⁰Th/²³²Th value (9.250; Robinson et al., 2004a), leading to larger corrections. Thorium incorporation from seawater can be considered a worst case scenario and is illustrated in Table T10 and Figure F11. Typically, where ²³²Th is high, crustal contamination is more likely, and therefore the correction for initial ²³⁰Th is small. Likewise, samples with lower ²³²Th, where incorporation of Th from seawater may be more significant, still have a small initial ²³⁰Th correction because of the low ²³²Th content. For both cases illustrated here, the initial ²³⁰Th has been shown to only be a significant correction for samples with ²³²Th > 5 ppb, and in the worst case scenario (i.e., incorporation of Th from seawater), the correction reduces ages by ~500 y with the exception of samples from Sections 325-M0032A-8R-CC and 325-M0033A-15R-CC.

Open system U-Th behavior

The [²³⁴U/²³⁸U]_i for all samples that yielded ages are close to the value of 1.147 for modern seawater and coral (Delanghe et al., 2002; Robinson et al., 2004a; Andersen et al., 2007). A clearly defined trend of lower initial ²³⁴U/²³⁸U with increasing age between 10 and 30 calibrated years before present (cal y BP; years before 1950 AD) is in agreement with previous observations (Yokoyama and Esat, 2004; Esat and Yokoyama, 2006). This agreement gives some qualitative indication of the reliability of the closed system assumption and, hence, the accuracy of the ages. There is, however, some variability in the [²³⁴U/²³⁸U]_i, which may be the result of the lower ²³⁴U/²³⁸U of seawater during glacial times (Yokoyama and Esat, 2004; Robinson et al., 2004b; Esat and Yokoyama, 2006, 2010) and/or some open system behavior of the corals.

Calibrated 14C ages

To account for variability in the ¹⁴C content of the atmosphere over time, radiocarbon ages have been calibrated using the marine09 calibration curve (Reimer et al., 2009) and a ΔR of 9 ± 45 (1σ) y, based on the mean and standard deviation of ΔR estimates from the Great Barrier Reef (GBR) region from the 14CHRONO marine reservoir database (intcal.qub.ac.uk/marine/). Calibration was performed using the OxCal v4.1 software (Bronk Ramsey, 2009). Results of this calibration are presented in Table T11. Where calibrated ¹⁴C ages have polymodal probability distributions (Fig. F12), the quoted 95.4% probability range has been taken from the youngest end of the youngest window to the oldest end of the oldest window, ignoring any intermediate gaps of low probability. For the purposes of assigning an age for qualitative comparison, the mean of the distribution is taken.

Some of the radiocarbon ages are near the limit of the ¹⁴C chronometer and thus may be much older than the raw radiocarbon age suggests. This is particularly true for the deep offshore Hole M0058A, which has an uncalibrated age of 48.0 ± 0.5 (1σ) ¹⁴C cal y BP for Section 325-M0058A-4R-2. This old age for the upper portion of this hole may be due to reworking and downslope transport of older material from the carbonate platform of the GBR to the continental slope, where it becomes incorporated in the younger sediment intersected by the drill core.

Summary

The positions of coral samples for which chronological control now exists have been plotted on the core log sections for each of the four sites. Where ages are referenced in this report, they have been rounded to the nearest 1 cal y BP. In all but one of the dated holes (Hole M0037A), the ages of the samples are in stratigraphic order, providing some confidence on the accuracy of the dates and that the cores have sampled in situ reef framework, consistent with the sedimentological observations. Further evidence for the quality of the U-Th ages is given by the limited variation in the [²³⁴U/²³⁸U]_i and its approximation to what one might expect for seawater during sea level lowstands. The quality of the preliminary U-Th data illustrates the promise for the development of an absolute chronology, where thorough screening will be performed to identify more pristine samples. This screening will be especially important to avoid detrital contamination, as was evident in the high ²³²Th concentrations in the samples dated here.

The majority of the deeper holes have U-Th dates near their bases that indicate the Last Glacial Maximum (LGM) intersected 10 samples between 20 and 25 cal y BP. Shallower samples from these cores (dated by radiocarbon) range to as recent as 13–14 cal y BP, indicating that the early portion of the deglacial has also been captured by Expedition 325 drill cores. Holes drilled in shallower water have ages as young as 10 cal y BP, which indicate that periods postdating MWP-1A (Fairbanks, 1989) have also been sampled, opening up the possibility of comparison of this event in the GBR with other localities. There appears to be a sharp decline in the number of samples postdating 10 cal y BP (Fig. F13). This may reflect a reef drowning event at this time or may simply be an artifact of the relatively small dataset and/or a sampling bias.

U-Th ages prior to the LGM show promise for recovering high-quality material for U-Th dating of earlier periods in the Pleistocene. Holes M0032A, M0056A, and potentially M0033A have material from marine isotope Stages 4 and 3, whereas Hole M0042A may provide material from the transition between marine isotope Stages 7 and 6. Hole M0057A, although not yielding a closed system U-Th age, has U-Th isotope ratios that suggest that the age of the samples is older than the LGM and, given its intermediate depth, may provide insight into glacial–interglacial transitions further back in time.

The offshore hole at Noggin Pass (Hole M0058A) produced a radiocarbon age close to the limit of the chronometer for a sample near the top of the hole. This suggests either a very slow sedimentation rate or that the sediment reaching this locality is reworked and hence much older than the age of sedimentation. If the latter is true, then establishing an accurate chronology for this core will not be possible using techniques that rely on the bulk isotopic compositions (¹⁴C and U-Th) of precipitated mineral phases. Other methods such as paleomagnetism, microfossil isotope stratigraphy, or tuning to orbital chronologies may be more successful.

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