PALEO-BREAKFAST, LUNCH, AND DINNER: ULTRA HIGH-RESOLUTION SAMPLING AND WHAT LIES BEHIND A SINGLE DRILL HOLE

Geochemical studies of seasonality in invertebrate carbonates rely on sub-sampling annual increments with a resolution of tens of samples per year. Each of these samples mixes shell material representing a few days to one month of shell growth and information about chemical variability on the daily-to-fortnightly scale is lost. This study compares stable isotope profiles in an intertidal bivalve shell at three different sampling resolutions, an annual cycle at fortnightly resolution, one fortnight at daily resolution and three days at multi-hourly resolution.

Daily growth bands in this bivalve (Chione cortezi, Gulf of California) are bundled into fortnight groups by tidally induced stresses. Sampling the central fast-growth carbonate of each fortnight bundle results in a smooth d¹⁸O curve ranging from –0.5‰ to –2.1‰. Adding samples at the boundaries of tidal bundles increased the noisiness of the curve dramatically, suggesting that there is significant d¹⁸O variation related to the fortnightly beats of the tides. Separating one tidal bundle into 10 samples revealed an initial decrease in d¹⁸O to –2.8‰ then an increase to +1.55 ‰ as the animal was stranded in an evaporating tide pool. The d¹⁸O amplitude within a fortnight is more than twice that of the smooth annual profile.

Dividing three days of growth into 15 samples revealed stable isotope variation that is at least 50% of the total annual amplitude. The daily cycle in d¹³C is at least 0.7‰. Subdaily sampling shows that the thin dark band (in reflected light) forms at the coolest temperatures and the thick white double-band forms during the day. Growth rates are highest during the day under open marine conditions and at night under stressed conditions. Carbon isotope minima occur mid-day and maxima are achieved in the late afternoon, a surprising pattern perhaps controlled by tidal flow.

Ultra high-resolution sampling demonstrates that there are many hierarchies of chemical variation in bivalve shells. Because of this, great care must be taken with sampling protocol in seasonal studies. The annual cycle in a shell may be obscured by short-term variation if small widely-spaced samples are taken. Simple d¹⁸O profiles do not necessarily demonstrate a simple temperature driven annual cycle.