Time in tortoiseshell: a bomb radiocarbon-validated chronology in sea turtle scutes
Kyle S.Van Houtan, Allen H.Andrews, T. ToddJones, Shawn K. K.Murakawa, Molly E.Hagemann
Published 6 January 2016.DOI: 10.1098/rspb.2015.2220
Kyle S. Van Houtan
NOAA Fisheries, Pacific Islands Fisheries Science Center, 1845 Wasp Boulevard, Building 176, Honolulu, HI 96818, USANicholas School of the Environment, Duke University, 450 Research Drive, Durham, NC 27708, USA
Interior structure of the posterior marginal scutes of hawksbill sea turtles contains an extensive chronology. (a) Adult female carapace measuring 83.2 cm straight length. Blue dashed outline is the PM scute, where the largest tissue record on the carapace resides. White-dashed line is the cross-section path. (b) Polished, composite image of the longitudinal cross-section of the PM scute identifying major features. Growth trajectory (old to new) runs left to right. The dark suture line that runs horizontally separates the dorsal/carapace and ventral/plastron portions of the shell. Parallel records occur on either side of this line. Shadows at individual image edges are peripheral halos from microscope field-of-view. Scute coloration varies naturally. (c) Growth line count for 36 hawksbills, obtained from the PM scute. A LOESS model fit through the data shows an expected growth curve form with three apparent stages (0–45 cm, 45–75 cm and greater than 75 cm) corresponding to known hawksbill stage classes. Shaded area is the 95% confidence band.
Hawaii coral cores provide a valid local bomb 14C reference series. (a) The Baker nuclear detonation at Bikini lagoon in 1946, a 23 kt Mark III ‘Fat Man’ plutonium core bomb was one of the first in a series of tests that reached yields in megatons of TNT. The dark shape in the central water pillar is the silhouette from the battleship USS Arkansas. Later mid-twentieth century atmospheric nuclear detonations dispersed large amounts of 14C across the biosphere, which accrued regionally at different rates and timelines. Image credit: US Government, public domain. (b) Sample sites in the Hawaiian archipelago providing Porites coral cores with date ranges. (c) Radiocarbon chronologies for Porites records. Black line is an ensemble model; dotted line is the linear extension through contemporary period.
Calibrating the hawksbill Δ14C curve from the coral record and validated hawksbill tissues. Owing to trophic effects, we expect the hawksbill Δ14C curve to be attenuated and potentially phase-lagged compared to corals. Filled circles are 14C values measured from the most recent keratin drill lines of 14 date-referenced Hawaii hawksbills, hollow circles are from three Hawaii captively reared hawksbills. Shaded area is the interval expected to account for all wild hawksbill observations.
Bomb radiocarbon dating of hawksbill scute growth line sequences. (a–c) Microsampled drill lines from individual scute cross sections provide Δ14C values throughout development, mapped onto the hawksbill Δ14C curve. Newest tissue was date-referenced, oldest tissue dates are limited VBGF constrains and Δ14C values, intermediary samples are interpolated. (d–o) Twelve cross-section images with sampled drill line transects. Measured lengths and lifespan dates are provided; scale bar for each inset represents 5 mm.
Growth model estimates for hawksbill age-at-maturity. (a) VBGF models fit to known ages of 16 captive-reared Pacific hawksbills places breeding maturity at 12.3 years (range 10–15 years), representing a potential ceiling of the somatic growth parameter, k, at 0.295. VBGF results considering (b) fastest, (c) mean and (d) slowest growth trajectories given the age range from the tissue Δ14C values, and extreme values of k. All panels use listed values for asymptotic length, L∞, and gestation time, t0, obtained from empirical monitoring, but fit k to the age estimates. Together mean results from these models estimate k is 0.13 (range 0.10–0.20) and age at maturity is 29 years (range 18–38). From these results, hawksbills deposit eight scute growth lines annually (range 5–14).