Siliceous productivity changes in Gulf of Ancud sediments (42°S, 72°W), southern Chile, over the last ∼150 years

We evaluated changes in siliceous export production and the source of organic matter preserved in sediment core MD07-3109H recovered from the Gulf of Ancud, Chiloe Inner Sea (42 degrees S, 72 degrees W, water column depth: 328 m), southern Chile. We analyzed the abundance of siliceous microfossils (diatoms, silicoflagellates, sponge spicules, Chrysophyte cysts, phytoliths), geochemical proxies (weight percent silicon %Si-OPAL, organic carbon, total nitrogen. C/N molar), and sediment stable isotopes (delta C-13(org), delta N-15). Chronology based on Pb-210 and C-14 provided an accumulated age of 144 years at the base of the core. Sediments of core MD07-3109H are predominantly marine in origin, averaging delta C-13(org)=-20.75 parts per thousand+0.82, delta N-15=8.7 +/- 0.35 parts per thousand, and C/N =8.76 +/- 0.36. Marine diatoms compose 94% of the total assemblage of siliceous microfossils. Our record of productivity based on the mass accumulation rates of organic carbon, total nitrogen, Si-OPAL, and total diatoms showed high values between 1863 and 1869 AD followed by a declining trend until 1921 AD, a transition period from 1921 to 1959 AD with fluctuating values, and a clear decreasing pattern from 1960 AD to the present. This marked reduction in productivity was associated with decreased precipitation and Puelo River streamflow (41 degrees S), as well as a warmer and more stratified water column, especially since the 1980s. (C) 2010 Elsevier Ltd. All rights reserved.