Antarctic Science 18 (1), 111-115 (2006) © Antarctic Science Ltd Printed in the UK DOI: 10.1017/S0954102006000101
Seasonal stable oxygen isotope cycles in an Antarctic bivalve shell (Laternula elliptica): a quantitative archive of ice-melt runoff
YOHEI TADA1, HIDEKI WADA2 and HIDEKI MIURA3 d ep a rtm en t o f Earth and Planetary Science, The University o f Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan 2Department o f Biology and Geosciences, Faculty o f Science, Shizuoka University, 836 Ova, Suruga-ku, Shizuoka, 422-8529, Japan division o f Geoscience, National Institute o f Polar Research (NIPR), 1-9-10 Kaga, Itabashi-ku, Tokyo, 173-8515, Japan ytada@eps. s. u-tokyo. ac.jp
Abstract: Oxygen isotope ratios in the shell of a Recent Antarctic marine bivalve Laternula elliptica are shown and their potential for environmental reconstruction is discussed. The shell 8180 profiles of this species represent the seasonal change in melted ice water inflow. Oscillations in the shell 8180 values reflect seasonal change in seawater 8180 values, caused by an addition of meltwater in summer. Since annual temperature variation is minimal and the inflow of the regional ice-melt is the dominant control on the shell 8180 values, the oxygen isotope record ofZ. elliptica is a quantitative indicator of the palaeo-ice-melting events in the Antarctic continental margin.
Received 4 January 2005, accepted 9 August 2005
Key words: Antarctica, bivalve shell, meltwater, seasonality, stable oxygen isotope
Introduction Berkman (1991) concluded that the low 8180 values of Oxygen isotope ratios in marine bivalve shells are well Adamussium colbecki (Smith, 1902), Antarctic scallops, are known to reflect both seawater temperatures and the caused by an inflow of the freshwater. Khim isotopic compositions of their habitats (Epstein et al. 1953). et al. (2001) suggested that the extremely low 8180 values These two independent factors must be combined to explain of Antarctic marine benthic foraminiferal tests, recovered the shell 8180 records of marine bivalves. In the seas around from two gravity cores in Maxwell and Admiralty bays, Antarctica, sea-surface temperature is fairly constant were caused by a significant decrease in the 8180 value of throughout the year (Ishii et al. 1998). Therefore, the seawater as a result of a meltwater discharge event. In isotopic composition of seawater should be the dominant comparison with homogenized bivalve shells and benthic control on shell 8180 values for Antarctic nearshore foraminiferal tests, a series of 8180 values from high- bivalves. resohition sequential sampling of Antarctic bivalve shells A latemulid bivalve species, Laternula elliptica (King & would be expected to have a much more detailed record of Broderip, 1831), is widely distributed in the Antarctic meltwater runoff, e.g. it would show the seasonal variation nearshore waters, burrowing deep into sediments and of marine salinity with a potential resolution of days to feeding on phytoplankton (Ahn & Shim 1998). Profiles of months. Fossil specimens ofZ. elliptica probably hold just shell carbon and oxygen isotope ratios of this species have such a record of the inflow of meltwater and the history of been documented by several workers. Barrera et al. (1994) ice sheet retreat in the Antarctic coastal region. constructed an oxygen isotopic thermometry equation based In this study, we present a much higher resolution 8180 on the analysis of 70 samples collected from right and left record from a single live-caught specimen of L. elliptica valves of a single specimen. Brey & Mackensen (1997) than those reported by previous authors and try to reconstructed the seasonal change of sea-surface reconstruct the present-day seasonal variation of meltwater temperatures based on the 8180 values of 43 samples inflow by comparing isotope records with the seasonal collected at intervals of 1 mm along the growth axis of a measurements in the region. In edition, we will try to single shell. However, the impact of changing seawater calculate the amount of meltwater inflow in this region and 8180 values was not taken into consideration in these compare it with the estimate by Berkman (1994) based on studies. Melted ice and/or snow (hereafter called the shell 8180 values. ‘meltwater’) flowing into the Antarctic continental margins during the southern hemisphere summer, is extremely M aterial and methods depleted in 180 (Craig 1961, Johnsen et al. 1972, Lorius et al. 1979, 1985, Matsubayaet al. 1979, Miura et al. 1998). A single Recent specimen, SUM-HM-W0007 ofZ. elliptica Inflow of meltwater into the sea should cause the seasonal was examined in this study (Fig. 1). It was collected alive lowering of the 8180 values around the Antarctic continental on Fukuro-ura Beach in Langhovde, the Soya Coast, the margin. Using the 8180 values of the homogenized shells, eastern part of Lützow-Holm Bay, East Antarctica (Fig. 2),
111 112 YOHEI TADA ef a/.
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Lanqhovda
Fukuro-ura
Fig. 1.The shell of Laternula elliptica (left valve) examined, showing the dorso-ventral line from which small amounts of carbonate samples were extracted successively for isotopic Skarvsnw analysis. Crests on the shell surface correspond to the hatched zones in Fig. 3. Scale bar = 1 cm. on 11 October 1985, during the 26th Japanese Antarctic Research Expedition (JARE-26, 1984-86). This specimen is one of numerous specimens which were transported ■Shaiififi; onshore by a recent storm; the cold temperatures preserved their fresh soft tissues (Murayama 1987). Both outer and inner shell layers of L. elliptica are composed entirely of aragonite, confirmed by X-ray diffraction analysis. For stable isotopic analysis, small Fig. 2. Sampling locality of the shell o fi. elliptica on Fukuro-ura amounts of powdered shell samples, c. 60.3 ± 26.4 |ig in Beach in Langhovde, the northern part of the Soya Coast. The weight, were carefully obtained from the external shell hatched zones indicate positions of seas. surface under a binocular microscope using a small knife. A series of samples (n — 101) were taken consecutively along the dorso-ventral axis (see Fig. 1). Each sample was 0.3 mm Results wide. The 8180 values of the shell analysed range from +2.66 to Isotopic measurements were made following the +4.79%o, and distinctly show a periodic fluctuation with 10 procedure described in Wada et al. (1984). Shell carbonate peaks (Fig. 3), in contrast to the 8180 profile of Brey & samples were reacted with pyro-phosphoric acid at 60.00°C Mackensen (1997). They required a better resolution to (Wada et al. 1984, modified from McCrea 1950) in an recognise the distinct wave pattern of 8180 values from the evacuated reaction system. All the isotopic compositions L. elliptica shell; our sampling resolution satisfies their were determined using a Finnigan MAT 250 mass requirement. The 8180 cycle lengths during the earlier spectrometer at Shizuoka University. The isotopic analyses periods of shell growth are longer than during the later ones. are reported in standard delta (6) notation in per mil units Brey & Mackensen (1997) also observed a similar change (%o) relative to the Pee Dee Belemnite (PDB) international in the 8180 wave lengths. This change in the 8180 standard (Craig 1957). The working reference standard CO, periodicity suggests that the shell growth rates of L. elliptica was calibrated against NBS19 and 20. Iterative gradually decrease during ontogeny. The amplitudes of the measurements of the working standard gas indicated that 8180 variations are almost constant during early growth. the standard error of the measurements is ± 0.04%o for During later growth, however, the most positive values of oxygen. This value was verified twice every day by each period are lower than that of the early growth. measurement of the in-house machine standard CO,. The two main factors controlling the oxygen isotope The soft tissue and the shell of this specimen are now composition of marine bivalve shells are temperature and housed in the Yokohama National University and the the 8180 value of the ambient seawater. In Lützow-Flolm Shizuoka University, respectively. Bay, bottom seawater temperature is almost constant LATERNULA ELLIPTICA OXYGEN ISOTOPE RATIOS 113 throughout a year, ranging from -1.37 to -1.91°C, with an annual average of -1.76°C (Fukuchi et al. 1985, Watanabe et al. 1986, Matsuda et al. 1987). In this study, we use the marine molluscan aragonite shell palaeo-thermometry relationship from Grossman & Ku (1986): cn 34 « T (°C) = 21.8 - 4.69 [S18Oarag - (S18Ow - 0.2)] ( 1 ) -t—'0 1 where T is the reconstructed seawater temperature, S18Oarag 03 m represents the stable oxygen isotope ratio of the molluscan 33 w T3 shell aragonite, and S18Ow is 8180 value of the local
Vmelt X SlsO melt + V seawater X SlsO seawater =tV k melt + V seawater' ) X SlsO W Í31V ' Thanks are due to M. Satish-Kumar (Shizuoka University), K. Kato (Tokyo University), H. Kitazato and T. Toyofuku where the volumes (V) and oxygen isotopic compositions (Japan Agency for Marine-Earth Science and Technology) (8180) of the meltwater streams (melt) and seawater for their advice. We also thank A.RM. Vaughan (editor), B. (seawater) are mixed (W). As mentioned above, the Schone (J.W. Goethe University), D. Dettman (University measured mean 8180 values of the meltwater and seawater of Arizona), and RA. Berkman (University of California) in this area are c. -30%o (Matsubaya et al. 1979) and -0.11 %o for valuable comments that improved the manuscript. (www.giss.nasa.gov ), respectively. As 818Ow, we use the most positive shell 8180 values on each cycle of the shell 8180 profile. The study area at Fukuro Cove was estimated References to be 1 km wide and 1.5 km across. In this study, we A h n , I.-Y. & S h im , J.H. 1998. Summer metabolism of the Antarctic clam, assumed the maximum water depth of our L. elliptica Laternula elliptica (King and Broderip) in Maxwell Bay, King George habitat as c. 10 m. The summer meltwater volume is Island and its implications. Journal o f Experimental Marine Biology and estimated as ranging from 0.6 x IO6 m3 to 1.0 x IO6 m3 at the Ecology, 224,253-264. highest. This estimation correlates well with that by B a r r e r a , E ., T e v e s z , M.J.S., C a r t e r , J.G. & M c C a l l , P.L. 1994. Oxygen and carbon isotopic composition and shell microstructure of bivalve Berkman (1994), who calculated the volume of glacial Laternula elliptica from Antarctica. Palaios, 9,275-287. meltwater runoff into summer Explorers Cove as ranging B e r k m a n , P.A. 1991. Holocene meltwater variations recorded in Antarctic from 0.59 x IO6 m3 to 0.95 x IO6 m3. And our calculated coastal marine benthic assemblages. In W e l l e r , G., W i l s o n , C.L. & range is valid for the observed summer freshwater volume S e v e r in , B. A.B., eds. International Conference on the Role o f the Polar of the Onyx River in the adjacent Dry Valleys (3.78 ± 3.49 x Regions in Global Changes. Fairbanks: University of Alaska, 440^149. B e r k m a n , P.A. 1994. Geochemical signatures of meltwater in mollusc IO6 m3, data from 1968 to 1988), the largest meltwater shells from Antarctic coastal areas during the Holocene. Memoirs of stream in Antarctica (Chinn 1993). National Institute o f Polar Research Special 50, Issue, 11-33. Our calculated volume of meltwater, however, may be B r e y, T. & M a c k e n s e n , A. 1997. Stable isotopes prove shell growth bands unreliable in the absence of various parameters; e.g. a in the Antarctic bivalve Laternula elliptica to be formed annually. Polar density gradient between seawater and meltwater, detailed Biology, 17, 465—468. C h in n , T. 1993. Physical hydrology of the Dry Valley lakes.Antarctic information of sea bottom topography, accurate positions of Research Series, 59, 1-52. L. elliptica habitats and meltwater streams, the volume of C r a i g , H. 1957. Isotopic standards for carbon and oxygen and correction groundwater seep (a part of meltwater), etc. We need better factors for the mass spectrometric analyses of carbon dioxide. constraints on these parameters to precisely estimate the Geochimica et Cosmochimica Acta, 12, 133-149. meltwater volume. LATERNULA ELLIPTICA OXYGEN ISOTOPE RATIOS 115
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