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Holocene biotic and abiotic marine calcite have a similar range of Mg contents (0 to 22 and 4 to 2 1 mol% MgC03, respectively), yet biotic calcite has Sr*+ concentrations that are consistently 1250 ppm higher than those of abiotic calcite. As in laboratory experiments, a positive linear relation is observed between Dsr and calcite Mg content. This produces two distinct linear trends on a plot of Sr2+ vs. Mg2+ concentrations. Principal axes of variation for both trends have similar slopes, yet distinctly different Sr'+ concentration intercepts. (Biotic: y = 0.024x + 1298, r2 = 0.70; Abiotic: y = 0.027x + 47, r2 = 0.77). The similar slopes of these trends reflect the constancy of Mg/Ca and Sr/Ca ratios of modem seawater. Equations describing the dependence of Ds* on calcite Mg content are derived from both trends (Biotic: Ds,,, = 3.16 X lo-@' (ppm Mg) + 0.169; Abiotic: Ds* = 3.52 X 10e6 (ppm Mg) + 0.0062). Characterization of Sr-Mg trends for Holocene materials allows comparison with analogous trends from ancient samples to estimate relative changes in seawater Mg/Ca and Sr/Ca ratios. The relatively high Sr contents of biotic calcite result from rapid p~~pi~tion rates associated with shell accretion in marine organisms. Calcites precipitated from seawater in laboratory experiments have Dsr values that are similar to those of biotic marine calcite, suggesting that both precipitate at approximately the same rate. Our estimates of surface area-normalized precipitation rates in planktonic and benthonic foraminifera are comparable to those of seeded, pH-stat experiments. We conclude that the Dsl values for biotic and experimental marine calcite are kinetically controlled, whereas the lower precipitation rates of abiotic marine calcite yield Dsl values that approximate equilibrium conditions. Experimentally derived equations describing the relation between Dsr and calcite precipitation rate indicate that the offset in Sr content between biotic and abiotic calcite is the result of abiotic precipitation rates that are two to five orders of magnitude lower than those of biotic precipitates. However, observations of naturally occurring marine cements suggest that the five-order-of-magnitude offset best represents natural system processes.

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OBJECTIVE:Preoperative localization of pancreatic neuroendocrine tumors with traditional imaging fails in 40 -60% of patients. Endoscopic ultrasound (EUS) is highly sensitive in the detection of these tumors. Previous reports included relatively few patients or required the collaboration of multiple centers. We report the results of EUS evaluation of 82 patients with pancreatic neuroendocrine tumors. METHODS:We prospectively used EUS early in the diagnostic evaluation of patients with biochemical or clinical evidence of neuroendocrine tumors. Patients had surgical confirmation of tumor localization or clinical follow-up of Ͼ1 yr. RESULTS:Eighty-two patients underwent 91 examinations (cases). Thirty patients had multiple endocrine neoplasia syndrome type I. One hundred pancreatic tumors were visualized by EUS in 54 different patients. The remaining 28 patients had no pancreatic tumor or an extrapancreatic tumor. Surgical/pathological confirmation was obtained in 75 patients. The mean tumor diameter was 1.51 cm and 71% of the tumors were Յ2.0 cm in diameter. Of the 54 explorations with surgical confirmation of a pancreatic tumor, EUS correctly localized the tumor in 50 patients (93%). Twentynine insulinomas, 18 gastrinomas, as well as one glucagonoma, one carcinoid tumor, and one somatostatinoma were localized. The most common site for tumor localization was the pancreatic head (46 patients). Most tumors were hypoechoic, homogenous, and had distinct margins. EUS of the pancreas was correctly negative in 20 of 21 patients (specificity, 95%). EUS was more accurate than angiography with or without stimulation testing (secretin for gastrinoma, calcium for insulinoma), transcutaneous ultrasound, and CT in those patients undergoing further imaging procedures. EUS was not reliable in localizing extrapancreatic tumors. CONCLUSIONS:In this series, the largest single center experience reported to date, EUS had an overall sensitivity and accuracy of 93% for pancreatic neuroendocrine tumors. Our results support the use of EUS as a primary diagnostic modality in the evaluation and management of patients with neuroendocrine tumors of the pancreas. (Am J Gastroenterol 2000;95:2271-2277.