NOAA/WDS Paleoclimatology - Fajardo, Puerto Rico Coral Carbon Isotope Data from 1948-2004 CE

Tropical small mountainous rivers deliver a poorly quantified, but potentially significant, amount of carbon to the world’s oceans. However, few historical records of land–ocean carbon transfer exist for any region on Earth. Corals have the potential to provide such records, because they draw on dissolved inorganic carbon (DIC) for calcification. In temperate systems, the stable- (d13C) and radiocarbon (Δ14C) isotopes of coastal DIC are influenced by the d13C and Δ14C of the DIC transported from adjacent rivers. A similar pattern should exist in tropical coastal DIC and hence coral skeletons. Here, d13C and Δ14C measurements were made in a 56-year-old Montastraea faveolata coral growing ~1 km from the mouth of the Rio Fajardo in eastern Puerto Rico. Additionally, the d13C and Δ14C values of the DIC of the Rio Fajardo and its adjacent coastal waters were measured during two wet and dry seasons. Three major findings were observed: (1) synchronous depletions of both d13C and Δ14C in the coral skeleton are annually coherent with the timing of peak river discharge, (2) riverine DIC was always more depleted in d13C and Δ14C than seawater DIC, and (3) the correlation of d13C and Δ14C was the same in both coral skeleton and the DIC of the river and coastal waters. These results indicate that coral skeletal d13C and Δ14C are recording the delivery of riverine DIC to the coastal ocean. Thus, coral records could be used to develop proxies of historical land–ocean carbon flux for many tropical regions. Such information could be invaluable for understanding the role of tropical land–ocean carbon flux in the context of land-use change and global climate change.

热带山地小型河流向全球海洋输送的碳量虽缺乏精准量化，但潜在贡献不容忽视。然而，全球任何区域的陆-海碳迁移历史记录都极为匮乏。珊瑚具备提供此类记录的潜力，因为其钙化作用所需的碳取自溶解无机碳（dissolved inorganic carbon, DIC）。在温带海域，沿岸溶解无机碳的稳定碳同位素（δ¹³C）与放射性碳同位素（Δ¹⁴C）特征，会受到周边河流输送的溶解无机碳的δ¹³C和Δ¹⁴C特征影响。热带沿岸溶解无机碳乃至珊瑚骨骼中，理应存在类似的同位素变化模式。本研究对采集自波多黎各东部法哈多河河口约1公里处的一株56年树龄的Montastraea faveolata珊瑚进行了δ¹³C与Δ¹⁴C同位素分析。此外，本研究在两个干湿季分别测定了法哈多河及其邻近沿岸水域溶解无机碳的δ¹³C与Δ¹⁴C数值。本研究得到三项核心结论：（1）珊瑚骨骼中δ¹³C与Δ¹⁴C的同步贫化现象，每年均与河流径流量峰值出现的时间相吻合；（2）河流来源的溶解无机碳的δ¹³C与Δ¹⁴C值始终较海水溶解无机碳更为贫化；（3）珊瑚骨骼与河流、沿岸水域溶解无机碳中的δ¹³C与Δ¹⁴C相关性保持一致。上述结果表明，珊瑚骨骼中的δ¹³C与Δ¹⁴C信号能够记录河流来源溶解无机碳向沿岸海洋的输送过程。因此，珊瑚同位素记录可用于构建多数热带区域的历史陆-海碳通量代用指标。这类信息对于理解热带陆-海碳通量在土地利用变化与全球气候变化背景下的作用，具有不可估量的价值。