Stoll Heather M, Ziveri Patricia, Geisen Markus, Probert Ian, Young Jeremy R
Geoscience Department, Williams College, Williamstown, MA 01267, USA.
Philos Trans A Math Phys Eng Sci. 2002 Apr 15;360(1793):719-47. doi: 10.1098/rsta.2001.0966.
The Sr/Ca ratio of coccoliths was recently proposed as a potential indicator of past growth rates of coccolithophorids, marine algae, which play key roles in both the global carbonate and carbon cycles. We synthesize calibrations of this proxy through laboratory culture studies and analysis of monospecific coccolith assemblages from surface sediments. Cultures of coccolithophorids Helicosphaera carteri, Syracosphaera pulchra and Algirospira robusta confirm a 1-2% increase in Sr/Ca per degrees C previously identified in Emiliania huxleyi and Gephyrocapsa oceanica. This effect is not due merely to increases in growth rate with temperature and must be considered in palaeoceanographic studies. In light-limited cultures of E. huxleyi, Calcidiscus leptoporus and G. oceanica at constant temperature, coccolith Sr/Ca ratios vary by 10% across the range of possible growth and calcification rates for a given species. Among different species under similar culture conditions, Sr/Ca ratios vary by 30%. Although the highest ratios are in the cells with highest calcification and organic carbon fixation rates, at lower rates there is much scatter, indicating that different mechanisms control interspecific and intraspecific coccolith Sr/Ca variations. In field studies in the Equatorial Pacific and Somalia coastal region, coccolith Sr/Ca correlates with upwelling intensity and productivity. A more dynamic response is observed in larger coccoliths like C. leptoporus (23-55% variation in Sr/Ca) than in smaller coccoliths of G. oceanica or Florisphaera profunda (6-15% variation in Sr/Ca). This response suggests that, despite temperature effects, coccolith Sr/Ca has potential as an indicator of coccolithophorid productivity. If the variable Sr/Ca response of different species accurately reflects their variable productivity response to upwelling (and not different slopes of Sr/Ca with productivity), coccolith Sr/Ca could provide useful data on past changes in coccolith ecology. The mechanism of coccolith Sr/Ca variations remains poorly understood but is probably more closely tied to biochemical cycles during carbon acquisition than to chemical kinetic effects on Sr incorporation in the calcite coccolith crystals.
颗石藻的锶钙比最近被提议作为颗石藻（海洋藻类）过去生长速率的一个潜在指标，颗石藻在全球碳酸盐和碳循环中都起着关键作用。我们通过实验室培养研究以及对表层沉积物中单一物种颗石组合的分析，综合了该代理指标的校准。对颗石藻卡特氏螺旋球石藻、美丽 Syracosphaera pulchra 和粗壮阿尔吉罗斯藻的培养证实，先前在赫氏艾氏藻和大洋桥石藻中发现的每摄氏度锶钙比增加 1 - 2%。这种效应不仅仅是由于生长速率随温度升高，在古海洋学研究中必须予以考虑。在赫氏艾氏藻、细弱钙盘石藻和大洋桥石藻在恒温下光照受限的培养中，对于给定物种，颗石锶钙比在可能的生长和钙化速率范围内变化 10%。在相似培养条件下的不同物种之间，锶钙比变化 30%。尽管最高比值出现在钙化和有机碳固定速率最高的细胞中，但在较低速率时存在很大的离散性，这表明不同机制控制着种间和种内颗石锶钙比的变化。在赤道太平洋和索马里沿海地区的实地研究中，颗石锶钙比与上升流强度和生产力相关。观察到像细弱钙盘石藻这样较大颗石的动态响应更明显（锶钙比变化 23 - 55%），而大洋桥石藻或深海弗洛里斯球石藻等较小颗石的响应则较小（锶钙比变化 6 - 15%）。这种响应表明，尽管存在温度效应，颗石锶钙比有潜力作为颗石藻生产力的指标。如果不同物种的锶钙比可变响应准确反映了它们对上升流的可变生产力响应（而不是锶钙比与生产力的不同斜率），颗石锶钙比可以提供有关过去颗石藻生态变化的有用数据。颗石锶钙比变化的机制仍知之甚少，但可能与碳获取过程中的生化循环关系更密切，而不是与方解石颗石晶体中锶掺入的化学动力学效应有关。
