Soil indicators of pre-European seabird breeding in New Zealand at sites identified by predator deposits

Abstract

Soil samples (0–15 cm) from 2 sites identified by palaeontological methods as pre-European seabird breeding sites (Annandale, Ardenest) were compared with a control site (Ngarua; no seabird breeding). All sites had moderately calcareous soils developed on limestone or marble. Breeding site Kjeldahl nitrogen (range, 0·41–1·4% N) and total phosphorus (range, 1780–5285 mg/kg P) were comparable to present-day breeding sites and higher than the control (mean, 0·20% N, 520 mg/kg P). Total cadmium (Cd) results followed the same pattern, with results from Ardenest (mean, 0·49 mg/kg) and Annandale (0·47 mg/kg) being similar to fertilised New Zealand pastoral soils. Contributions from extensive pre-European seabird breeding may therefore match superphosphate fertiliser as a Cd source. Results for δ¹⁵N and C: N were consistent with seabird breeding at both sites (Ardenest, δ¹⁵N = 8·4 , C:N = 10·6 g C/g N; Annandale, δ¹⁵N = 6·9 , C: N = 10·6). Results for δ¹³C were consistent with seabird breeding at Ardenest (δ¹³C = –22–8 ) but not at Annandale (δ¹³C = –27·0 ), indicating dilution by organic matter derived from C₃ plants at Annandale. The Cd : P ratio was significantly lower (P = 0·05) at each of the seabird breeding sites (Annandale, 6·5 ± 1·5 x 10^-5 mol Cd/mol P; Ardenest, 2·5 ± 0·7 x 10^-5) than the control (mean 10·0 ± 1·6 x 10^-5). This ratio has the potential to complement δ¹⁵N, δ¹³C, and C: N in identifying and mapping pre-European seabird breeding sites, although more study is required.