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

Abstract

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.

Nitrogen mineralization from organic fertilizers and composts: Literature survey and model fitting

“…The model calculations assumed optimal moisture content and a constant temperature of 25 ˚C when incorporated into warm and moist soil. The organic N fractions in feather meal and guano consist mainly of protein and uric acid, respectively (Lucassen et al, 2017;Papadopoulos, 1985), both of which are readily mineralized in soil, as our analysis shows. A slightly larger proportion of the total N in guano becomes plant available within 100 d of incorporation into warm and moist soil (70-75%) than for feather meal (50-65%).…”

The influence of seabirds on their breeding, roosting and nesting grounds: A systematic review and meta‐analysis

“…They can easily trace seabird‐derived nutrients in coastal and island food webs because of the distinction in marine δ 15 N and δ 13 C values compared to terrestrial N and C isotopes (Harding et al, 2004). The δ 15 N values of consumers are enriched by 3–5‰ with each trophic transfer along the marine food web (Post, 2002), thus seabirds—being mostly tertiary consumers—generally have high δ 15 N values (Lucassen et al, 2017). As guano contains considerable amounts of marine‐derived N, stable isotope analyses can be applied to materials (e.g.…”

“…While it is important to study all species, regardless of population status, it is concerning that very few studies have examined globally threatened species, given the significant influence seabirds have on their terrestrial environments (Rodrigues et al, 2021). Interestingly, seabirds that were historically important guano‐producing birds (whose guano was mined for fertilizer) in South America were only studied once (Peruvian Booby, Sula variegata ; Lucassen et al, 2017), despite two of the species (Guanay cormorant Phalacrocorax bougainvillii and the Peruvian brown pelican Pelecanus thagus ) being listed as Near Threatened on the Red List. The guano produced by these species is evidently laden with nutrients (Szpak, Millaire, et al, 2012), thus a decline in their populations would lead to changes to their habitats, yet as these species have not been studied, we do not know what these changes would be.…”

“…Humans have been aware of the many benefits of seabird guano and have exploited it for the purposes of fertilizing crops as early as the 16th century (Cushman, 2013). The most well‐known areas of guano mining were along the Pacific Margin in Peru and northern Chile where thick deposits of guano (>50 m in extreme cases) were mined extensively from islands (Lucassen et al, 2017; Szpak, Longstaffe, et al, 2012). The trade of guano peaked at 20 million tons/year during the mid‐19th century before declining as guano ran low, prices soared and chemical fertilizers became widely available (Mathew, 1970; Szpak, Longstaffe, et al, 2012).…”

‘White gold’ guano fertilizer drove agricultural intensification in the Atacama Desert from ad 1000

“…Seabird guano has conspicuously high δ 15 N values reflecting the birds' consumption of 15 N-enriched fish, which is further strongly increased by aerobic decomposition and subsequent volatilization of ammonia 22 . This leads to extremely high δ 15 N values for rookery deposits off Antarctica 22,23 and northern Chile (23.0 ± 8.4‰) 24 . Based on this information, we argue that the high crop δ 15 N values could only have been caused by the use of seabird guano as a fertiliser.…”