Stable isotope ratios in shells of marine organisms

Oxygen and carbon isotope analyses have been carried out on calcareous skeletons of important recent groups of organisms. Annual temperature ranges and distinct developmental stages can be reconstructed from single shells with the aid of the micro-sampling technique made possible by modern mass-spectrometers. This is in contrast to the results of earlier studies which used bulk sampIes. The skeletons analysed are from Bermuda, the Philippines, the Persian Gulf and the continental margin off Peru. In these environments, seasonal salinity ranges and thus annual variations in the isotopic composition of the water are small. In addition, environmental parameters are weIl documented in these areas. The recognition of seasonal isotopic variations is dependant on the type of calcification. Shells built up by carbonate deposition at the margin, such as molluscs, are suitable for isotopic studies. Analysis is more difficult where chambers are added at the margin of the shell but where older chambers are simultaneously covered by a thin veneer of carbonate e. g. in rotaliid foraminifera. Organisms such as calcareous algae or echinoderms that thicken existing calcareous parts as weIl as growing in length and breadth are the most difficult to analyse. All organisms analysed show temperature related oxygen-isotope fractionation. The most recent groups fractionate oxygen isotopes in accordance with established d18O temperature relationships (Tab. 18, Fig. 42). These groups are deep-sea foraminifera, planktonic foraminifera, serpulids, brachiopods, bryozoa, almost all molluscs, sea urchins, and fish (otoliths). A second group of organisms including the calcareous algae Padina, Acetabularia, and Penicillus, as weIl as barnacles, cause enrichment of the heavy isotope 18O. Finally, the calcareous algae Amphiroa, Cymopolia and Halimeda, the larger foraminifera, corals, starfish, and holothurians cause enrichment of the lighter isotope 16O. Organisms causing non-equilibrium fractionation also record seasonal temperature variations within their skeletons which are reflected in stable-oxygen-isotope patterns. With the exception of the green algae Halimeda and Penicillus, all organisms analysed show lower d13C values than calculated equilibrium values (Tab. 18, Fig. 42). Especially enriched with the lighter isotope 12C are animals such as hermatypic corals and larger foraminifera which exist in symbiosis with other organisms, but also ahermatypic corals, starfish, and holothurians. With increasing age of the organisms, seven different d13C trends were observed within the skeletons. 1) No d13C variations are observed in deep-sea foraminifera presumably due to relatively stable environmental conditions. 2) Lower d13C values occur in miliolid larger foraminifera and are possibly related to increased growth with increasing age of the foraminifera. 3) Higher values are found in planktonic foraminifera and rotaliid larger foraminifera and can be explained by a slowing down of growth with increasing age. 4) A sudden change to lower d13C values at a distinct shell size occurs in molluscs and is possibly caused by the first reproductive event. 5) A low-high-Iow cycle in calcareous algae is possibly caused by variations in the stage of calcification or growth. 6) A positive correlation between d18O and d13C values is found in some hermatypic corals, all ahermatypic corals, in the septa of Nautilus and in the otoliths of fish. In hermatypic corals from tropical areas, this correlation is the result of the inverse relationship between temperature and light caused by summer cloud cover; in other groups it is inferred to be due to metabolic processes. 7) A negative correlation between d18O and d13C values found in hermatypic corals from the subtropics is explained by the sympathetic relationship between temperature and light in these latitudes. These trends show that the carbon isotope fractionation is controlled by the biology of the respective carbonate producing organisms. Thus, the carbon isotope distribution can provide information on the symbiont-host relationship, on metabolic processes and calcification and growth stages during ontogenesis of calcareous marine organisms.本研究针对重要现生生物类群的钙质骨骼（calcareous skeletons）开展了氧、碳同位素分析。借助现代质谱仪（mass-spectrometers）支撑的微取样技术，可从单个壳体中重建年度温度区间与明确的发育阶段，这与早期采用批量样品（bulk samples）的研究结果形成鲜明对比。本次分析的骨骼样品采自百慕大、菲律宾、波斯湾以及秘鲁外陆缘海域。上述环境的季节性盐度区间及水体同位素组成的年度变化均较小，且各环境参数在这些区域均有详实的记录。 季节尺度同位素变化的识别取决于钙化作用（calcification）类型。以壳体边缘碳酸盐沉积方式生长的壳体（如软体动物（molluscs））适用于同位素研究；若壳体在边缘新增房室的同时，旧房室被薄层碳酸盐覆盖（如轮孔虫类有孔虫（rotaliid foraminifera）），则分析难度更高；而诸如钙质藻类（calcareous algae）、棘皮动物（echinoderms）这类既增厚现有钙质结构，同时又沿长宽方向生长的生物，其壳体的同位素分析难度最大。 所有分析的生物均表现出与温度相关的氧同位素分馏效应。其中多数类群的氧同位素分馏符合已确立的δ¹⁸O（d18O）-温度关系（表18、图42），这类生物包括深海有孔虫、浮游有孔虫、龙介虫（serpulids）、腕足动物（brachiopods）、苔藓虫（bryozoa）、几乎所有软体动物、海胆（sea urchins）以及鱼类耳石（otoliths）。第二类生物包括钙质藻类团扇藻属（Padina）、伞藻属（Acetabularia）、笔藻属（Penicillus）以及藤壶（barnacles），这类生物会富集重同位素¹⁸O。最后一类生物包括钙质藻类叉珊藻属（Amphiroa）、拟仙掌藻属（Cymopolia）、钙扇藻属（Halimeda）、大型有孔虫（larger foraminifera）、珊瑚（corals）、海星（starfish）以及海参（holothurians），这类生物会富集轻同位素¹⁶O。即便发生非平衡分馏（non-equilibrium fractionation）的生物，其骨骼内也记录了季节性温度变化，这一变化体现在稳定氧同位素模式（stable-oxygen-isotope patterns）中。 除绿藻钙扇藻属（Halimeda）和笔藻属（Penicillus）外，所有分析生物的δ¹³C（d13C）值均低于计算得到的平衡分馏值（表18、图42）。轻同位素¹²C富集程度最高的类群包括与其他生物共生的造礁珊瑚（hermatypic corals）、大型有孔虫，以及非造礁珊瑚（ahermatypic corals）、海星和海参。随着生物个体年龄增长，在其骨骼中可观测到7种不同的δ¹³C变化趋势： 1) 深海有孔虫未观测到δ¹³C变化，推测是因为其生存环境相对稳定。 2) 粟虫类有孔虫（miliolid foraminifera）的δ¹³C值较低，可能与其随年龄增长而加快的生长速率相关。 3) 浮游有孔虫与轮孔虫类大型有孔虫的δ¹³C值较高，这可通过其随年龄增长而放缓的生长速率来解释。 4) 软体动物的δ¹³C值在壳体达到特定大小时会突然降低，这可能与其首次繁殖事件（reproductive event）有关。 5) 钙质藻类的δ¹³C值呈现低-高-低的循环模式，这可能与钙化作用或生长阶段的变化相关。 6) 部分造礁珊瑚、所有非造礁珊瑚、鹦鹉螺的隔片（septa）以及鱼类耳石中，δ¹⁸O与δ¹³C值呈正相关关系（positive correlation）。在热带区域的造礁珊瑚中，这种相关性源于夏季云量导致的温度与光照的负相关关系；在其他类群中，该相关性被认为与代谢过程相关。 7) 亚热带区域的造礁珊瑚中，δ¹⁸O与δ¹³C值呈负相关关系（negative correlation），这可通过该纬度带温度与光照的协同变化关系来解释。 上述趋势表明，碳同位素分馏由各自产碳酸盐生物的生物学特性所调控。因此，碳同位素分布可为海洋钙质生物的共生宿主关系、代谢过程、钙化作用以及个体发育（ontogenesis）过程中的生长阶段提供相关信息。