Latitudinal variations in delta(30)Sj and delta N-15 signatures along the Peruvian shelf: quantifying the effects of nutrient utilization versus denitrification over the past 600 years

摘要

The stable sedimentary nitrogen isotope compositions of bulk organic matter (delta N-15(buik)) and the silicon isotope composition of diatoms (delta Si-30(Bsi)) both mainly reflect the degree of past nutrient utilization by primary producers. However, in ocean areas where anoxic and suboxic conditions prevail, the delta N-15(buik) signal ultimately recorded within the sediments is also influenced by water column denitrification, causing an increase in the subsurface delta N-15 signature of dissolved nitrate (delta(NO3)-N-15) upwelled to the surface. Such conditions are found in the oxygen minimum zone off the coast of Peru, where, at present, an increase in subsurface delta 15NO; from north to south along the shelf is observed due to ongoing denitrification within the poleward-flowing subsurface waters, while the delta Si-30 signature of silicic acid (delta Si-30(OH)(4)) at the same time remains unchanged. Here, we present three new delta(30)SiBs, records between 11 and 15 S and compare these to previously published delta Si-30(Bs), and delta N-15(bulk) records from Peru covering the past 600 years. We present a new approach to calculate past subsurface 815NO; signatures based on the direct comparison of delta Si-30(Bs), and delta N-15(bulk) signatures at a latitudinal resolution for different time periods. Our results show that, during the Current Warm Period (CWP, since 1800 CE) and prior shortterm arid events, source water delta(NO)-N-15; compositions have been close to modern values, increasing southward from 7 to 10%0 (between 11 and 15 S). In contrast, during the Little Ice Age (LIA) we calculate low delta(NO3)-N-15 values between 6 parts per thousand and 7.5 parts per thousand. Furthermore, the direct delta Si-36(Bs), versus delta N-15(bulk) comparison also enables us to relate the short-term variability in both isotope compositions to changes in the ratio of nutrients (NO3- : Si(OH)(4)) taken up by different dominating phytoplankton groups (diatoms and non-s