A STABLE ISOTOPE INVESTIGATION OF CALCAREOUS TUBES FROM RECENT SERPULID AGGREGATES IN BAFFIN BAY, TEXAS

To better understand the origin of large serpulid aggregates, this study examines isotopic compositions of aragonitic serpulid tubes from Baffin Bay, Texas. This hypersaline lagoon has a small tidal range, 1.5m average depth, 10°-30°C water temperatures, and salinity that averages 40-60‰ and ranges 0-100‰. Serpulid aggregates are rarely exposed and form patch reefs at the bay mouth and larger reef fields within the bay in 0.5 to 2.5m deep waters. Reef fields consist of randomly intertwined tubes, whereas patch reefs have alternating layers of random and oriented tubes. Oriented tubes are nearly straight, vertical, up to 3cm long, with diameters of 1-2mm, and walls 0.2 to 0.3mm thick. Random aggregates have smaller, curved tubes. Previous radiocarbon dating by others indicates serpulid growth within the last 3,000 years.

Isotopic compositions of oriented tubes vary substantially (d¹⁸O=-4.9 to +0.6‰; d¹³C=-4.1 to -0.3‰; VPDB), indicating growth under variable conditions. The most negative values correspond to samples from the base of oriented layers, suggesting that their initiation may have been promoted by lowered salinity and enhanced input of terrestrial carbon during short intervals of increased freshwater input. During formation of oriented aggregates salinity fluctuates, but the water becomes, in general, more saline. This is suggested by variable compositions of individual tube segments and by isotopic enrichment of many samples from the uppermost parts of oriented layers. The overlapping compositions of random tubes from the patch reefs and the reef fields suggest similar conditions during their formation. Their d¹³C values (-4.7 to -0.8‰) are similar to those of the oriented tubes, but the random tubes have a narrower d¹⁸O range (-1.9 to +0.5‰). This reflects a lower environmental variability and suggests that even though serpulids can grow under extremely variable conditions, the aggregates of randomly oriented tubes likely form by periodic rapid accumulation of numerous individuals during optimal growth conditions.