Ecological Archives E093-011-A1

S. Uthicke, F. Patel, and R. Ditchburn. 2012. Elevated land runoff after European settlement perturbs persistent foraminiferal assemblages on the Great Barrier Reefs. Ecology 93:111–121.

Appendix B. Methods and results of sediment and foraminifera dating.

¹³⁷Cs (half-life = 30.2 years), from atmospheric nuclear weapons testing, and²¹⁰Pb (half-life = 22.3 years), produced by the decay of atmospheric ²²²Rn, are widely used for dating marine sediment and inferring sedimentation rates. Principles of the method and the chemical procedure for measuring ²¹⁰Pb, via its grand-daughter ²¹⁰Po, were given by Graham et al. (2005).

Carbonate was separated from the silicate on which ¹³⁷Cs and ²¹⁰Pb are adsorbed.  The ultra fine silicate and pulverized shell was washed from the carbonate and left to settle for several days.  Excess water was decanted, the silt dried at 40 °C and then crushed.

¹³⁷Cs was analyzed by direct counting of the sediment samples in a high-resolution low-background germanium (well-crystal) gamma detector with anticoincidence shielding.

²¹⁰Pb was analyzed by extracting the granddaughter ²¹⁰Po and counting it in an alpha spectrometer. Both ¹³⁷Cs and ²¹⁰Pb were analyzed at GNS Science, National Isotope Centre, NZ. ²¹⁰Pb activities were expressed as concentrations in the silicate. 

Sedimentation rates established using ¹³⁷Cs were expressed as a range based on the upper and lower depth limits for where ¹³⁷Cs started to increase from the low levels c. 1955.  Due to delayed input from the land, the ¹³⁷Cs concentration increased during this period and reached a plateau sometime after the first sediment with ¹³⁷Cs from the 1964–1965 fallout maximum had been deposited Pfitzner et al. (2004). The depth at which this occurred was presumed to be near the bottom of the layer in which the plateau was reached. Sedimentation rates based on ²¹⁰Pb and ¹³⁷Cs were calculated from the upper slices of the cores to the depth where no more excess ²¹⁰Pb could be determined (after 6–7 half-lives, i.e., about 150 years). Samples were dated from one core per reef; duplicate cores were analyzed from Deloraine Island.

Carbon dating was conducted directly on foraminifera from 17 of the deeper core slices. To achieve this several cleaned specimens of the foraminifera were pooled. Most ¹⁴C analyses were conducted on Elphidium spp. (pooled E. craticulatum and E. crispum), however, specimens of Amphistegina lessonii were used from several slices. To test for comparability between the two species we analyzed both taxa from two core slices in parallel. After pre-treatment as described in Lowe et al. (1988), purified CO₂ was converted to graphite by heating to 700°C with hydrogen over a Fe catalyst before measurement using the Accelerator Mass Spectrometer facility at the GNS Science National Isotope Centre, Lower Hutt, NZ. Conventional carbon dates were calculated using a marine reservoir value (delta-R) of +10 ± 7 which represents the current best estimate for the Queensland coast.

Results

Using the ²¹⁰Pb method, sedimentation rates during the past ~ 150 yr were established for each location.  In most cores, the excess ²¹⁰Pb versus depth decay curve was exponential, and the supported ²¹⁰Pb background remained constant. Although Deloraine Island duplicate cores 5 and 11 had only three and two layers respectively with excess ²¹⁰Pb, their estimated sedimentation rates were in agreement. Some other cores exhibited fluctuations in sedimentation rate and overall rates given per core represent averages (Table 1). Sedimentation rates thus estimated ranged from 0.20 to 0.58 cm year^-1, with no obvious spatial pattern in sedimentation rates, e.g., between inshore and outer island reef cores.

 ¹³⁷Cs data confirmed ages derived using ²¹⁰Pb for the upper sections of each core. Fallout from nuclear weapons testing started to increase from low levels ~ 1953. The exact depth where ¹³⁷Cs became detectable could not be determined because of the thickness of the core slices (10cm).  However, this has no affect on the grouping of ages for foraminiferal assemblage analysis because the sediment containing measurable amounts of ¹³⁷Cs must be less than 55 years old.

Foraminiferal samples from several cores were carbon dated. Maximum conventional radiocarbon ages determined were 3400 years corresponding to a calibrated age of 2989 to 3462 years BP. The youngest samples which could be reliably dated, as judged by a small 68.5% probability interval (one sample with a large interval was omitted from further analysis), were derived from foraminifera from sediment at a depth of 30–40 cm from Dent Island and Deloraine Island; these foraminifera were estimated to be from early in the 20^th century. Both foraminiferal groups used for ¹⁴C analysis were analyzed separately in two core slices from Lindeman Island. Although ages estimated from Elphidium spp. in one slice were slightly older than those derived from A. lessonii, samples ages were in the same range.

Age data derived from ¹⁴C, and ²¹⁰Pb and ¹³⁷Cs were compared in Fig. B1. Three of the four samples where data can be compared directly show close matches between the methods. However, in one sample (50–60 cm in Core 12b), ages derived from ¹⁴C in foraminifera (average: 453 yr) were older than the sediment age as suggested by ²¹⁰Pb (130–150 yr). In three of the eight cores (11, 5, 12) where samples were dated using both methods, apparent sediment accumulation rates match between shallow slices (²¹⁰Pb and ¹³⁷Cs) and deeper ones (dated by ¹⁴C). In the remaining cores sediment accumulation were lower for deeper core slices.

Table B1. Results of ²¹⁰Pb and ¹³⁷Cs analysis of the upper slices of sediment cores. The sedimentation rate for each core was estimated from the plot of excess ²¹⁰Pb versus depth. Grouping of slices into age bands is based on estimated sedimentation rates, supplemented with information on ¹³⁷Cs and ²¹⁰Pb: If ¹³⁷Cs is detectable, sediments are presumed to be < 55years old. If ²¹⁰Pb but no ¹³⁷Cs is detectable, sediments are between 55 and 150 years old. Sediments without detectable ²¹⁰Pb are older than 150 years. * indicates incongruously low ²¹⁰Pb_excess values or those too close to supported ²¹⁰Pb background activity - data omitted from sedimentation rate estimation. Abbreviations: n.d.: ¹³⁷Cs below detection limit, n.u.: sediment slice not grouped because not used for foraminiferal assemblage analyses.

Table B2. Results of the ¹⁴C analysis of foraminiferal samples from the Great Barrier Reef. Calibrated age (average) is given based on 68.5% probability intervals (in brackets) and calibration using a reservoir value of +10 ± 7 (Ulm 2002). Species abbreviations: Ec: Elphidium craticulatum,  E. spp.: sample consists of specimens from E. craticulatum and E. crispum, Al: Amphistegina lessonii.

Fig. B1. Summary of sediment core dating of eight cores (C) retrieved from fringing coral reefs in the Whitsunday area of the Great Barrier Reef, Australia. Details and error values are given in Tables B1 and B2. Filled squares represent ages based on ¹³⁷Cs and ²¹⁰Pb dating (Table B1), open squares are ages derived from ¹⁴C dating of foraminifera (Table B2).

Literature Cited

Graham, I. J., B. V. Alloway, U. A. Cochran, R. A. Cook, R. G. Ditchburn, D. C. Mildenhall, U. Morgenstern, and C. A. Prior. 2005. Sedimentology, geochronology and micropaleontology of post- and immediately pre-European Lake Tekapo sediment (based on analysis of core L1395) Lower Hutt: Institute of Geological & Nuclear Sciences Limited. Institute of Geological & Nuclear Sciences science report 2005/34. 82p.

Lowe, D., G. Brailsford, and G. Drummond. 1988. Microwave oven pretreatment of carbonates for 14C dating by accelerator mass spectrometry. Radiocarbon 30:385–387.

Perry, C., S. Smithers, and K. Johnson. 2009. Long-term coral community records from Lugger Shoal on the terrigenous inner-shelf of the central Great Barrier Reef, Australia. Coral Reefs 28:941–948.

Pfitzner, J., G. Brunskill, and I. Zagorskis. 2004. 137Cs and excess 210Pb deposition patterns in estuarine and marine sediment in the central region of the Great Barrier Reef Lagoon, north-eastern Australia. Journal of environmental radioactivity 76:81–102.

Ulm, S. 2002. Marine and estuarine reservoir effects in central Queensland, Australia: Determination of R values. Geoarchaeology 17:319–348.

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