The stable isotope composition of nitrogen and carbon and elemental contents in modern and fossil seabird guano from Northern Chile – Marine sources and diagenetic effects

Seabird excrements (guano) have been preserved in the arid climate of Northern Chile since at least the Pliocene. The deposits of marine organic material in coastal areas potentially open a window into the present and past composition of the coastal ocean and its food web. We use the stable isotope composition of nitrogen and carbon as well as element contents to compare the principal prey of the birds, the Peruvian anchovy, with the composition of modern guano. We also investigate the impact of diagenetic changes on the isotopic composition and elemental contents of the pure ornithogenic sediments, starting with modern stratified deposits and extending to fossil guano. Where possible, 14C systematics is used for age information. The nitrogen and carbon isotopic composition of the marine prey (Peruvian anchovy) of the birds is complex as it shows strong systematic variations with latitude. The detailed study of a modern profile that represents a few years of guano deposition up to present reveals systematic changes in nitrogen and carbon isotopic composition towards heavier values that increase with age, i.e. depth. Only the uppermost, youngest layers of modern guano show compositional affinity to the prey of the birds. In the profile, the simultaneous loss of nitrogen and carbon occurs by degassing, and non-volatile elements like phosphorous and calcium are passively enriched in the residual guano. Fossil guano deposits are very low in nitrogen and low in carbon contents, and show very heavy nitrogen isotopic compositions. One result of the study is that the use of guano for tracing nitrogen and carbon isotopic and elemental composition in the marine food web of the birds is restricted to fresh material. Despite systematic changes during diagenesis, there is little promise to retrieve reliable values of marine nitrogen and carbon signatures from older guano. However, the changes in isotopic composition from primary marine nitrogen isotopic signatures towards very heavy values generate a compositionally unique material. These compositions trace the presence of guano in natural ecosystems and its use as fertilizer in present and past agriculture.

海鸟粪（guano）至少自上新世（Pliocene）起，便在智利北部的干旱气候中得以保存。沿海区域的海洋有机质（marine organic material）沉积物，为探究近海及其食物网的古今组成提供了潜在契机。本研究通过测定氮（nitrogen）、碳（carbon）稳定同位素组成（stable isotope composition）与元素含量（element contents），将鸟类的主要猎物——秘鲁鳀（Peruvian anchovy）与现代鸟粪的组成进行对比分析。同时，我们以现代层状沉积物（modern stratified deposits）为起始，延伸至化石鸟粪（fossil guano），探讨成岩变化（diagenetic changes）对纯鸟源性沉积物（pure ornithogenic sediments）的同位素组成及元素含量的影响。在条件允许时，采用碳14（14C）测年体系获取年代信息。

鸟类的海洋猎物秘鲁鳀的氮、碳稳定同位素组成较为复杂，其随纬度（latitude）呈现显著的系统性变化。对代表数年现代鸟粪沉积直至现今的现代剖面开展的详细研究显示，氮、碳稳定同位素组成随沉积年龄（即深度）增加，呈现向偏重同位素值的系统性偏移。仅现代鸟粪最上层、最年轻的层位，其组成与鸟类猎物的组成具有相似性。在该剖面中，脱气作用（degassing）导致氮与碳同步流失，而磷（phosphorous）、钙（calcium）等非挥发性元素（non-volatile elements）则在残留鸟粪中发生被动富集。

化石鸟粪沉积物的氮含量极低，碳含量亦偏低，且氮同位素组成偏重。本研究的一项核心结论为：利用鸟粪示踪鸟类海洋食物网的氮、碳同位素及元素组成，仅适用于新鲜样品。尽管成岩变化会引发系统性分异，但从较老的鸟粪中获取可靠的海洋氮、碳同位素特征的可行性极低。不过，由原始海洋氮同位素特征向偏重同位素值转变所形成的独特组成，可用于示踪自然生态系统中鸟粪的存在，以及古今农业中鸟粪作为肥料的应用情况。