Siliceous Productivity Changes in Gulf of Ancud Sediments (421S, 721W), Southern Chile, Over the Last 150 Years
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Continental Shelf Research 31 (2011) 356–365 Contents lists available at ScienceDirect Continental Shelf Research journal homepage: www.elsevier.com/locate/csr Siliceous productivity changes in Gulf of Ancud sediments (421S, 721W), southern Chile, over the last 150 years Lorena Rebolledo a,b,n, Humberto E. Gonza´lez a,b,c,d, Pra´xedes Mun˜oz e, Jose´ L. Iriarte c,d,f, Carina B. Lange b,c,d,g, Silvio Pantoja b,c,d,g, Marco Salamanca g a Instituto de Biologı´a Marina, Facultad de Ciencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile b Centro de Investigacio´n Oceanogra´fica en el Pacı´fico Sur-Oriental (COPAS), Universidad de Concepcio´n, Casilla 160-C, Concepcio´n, Chile c Programa COPAS Sur-Austral, Universidad de Concepcio´n, Concepcio´n, Chile d Centro de Investigacio´n en Ecosistemas de la Patagonia (CIEP), Bilbao 449, Coyhaique, Chile e Departamento de Biologı´a Marina, Facultad de Ciencias del Mar y Centro de Estudios en Zonas A´ridas (CEAZA), Universidad Cato´lica del Norte, Larrondo 1281, Coquimbo, Chile f Instituto de Acuicultura, Centro de Investigacio´n en Nutricio´n (CIEN), Universidad Austral de Chile, Casilla 1327, Puerto Montt, Chile g Departamento de Oceanografı´a, Universidad de Concepcio´n, Casilla 160-C, Concepcio´n, Chile article info abstract Article history: We evaluated changes in siliceous export production and the source of organic matter preserved in Received 17 November 2009 sediment core MD07-3109H recovered from the Gulf of Ancud, Chiloe´ Inner Sea (421S, 721W, water Received in revised form column depth: 328 m), southern Chile. We analyzed the abundance of siliceous microfossils (diatoms, 16 June 2010 silicoflagellates, sponge spicules, Chrysophyte cysts, phytoliths), geochemical proxies (weight percent Accepted 23 June 2010 silicon %Si , organic carbon, total nitrogen, C/N molar), and sediment stable isotopes (d13C , d15N). Available online 21 July 2010 OPAL org Chronology based on 210Pb and 14C provided an accumulated age of 144 years at the base of the core. Keywords: 13 Sediments of core MD07-3109H are predominantly marine in origin, averaging d Corg¼ Productivity –20.75%70.82, d15N¼8.770.35%, and C/N¼8.7670.36. Marine diatoms compose 94% of the total Siliceous microfossil assemblage of siliceous microfossils. Our record of productivity based on the mass accumulation rates Geochemical proxies of organic carbon, total nitrogen, Si , and total diatoms showed high values between 1863 and 1869 Sediments OPAL Gulf of Ancud AD followed by a declining trend until 1921 AD, a transition period from 1921 to 1959 AD with Chile 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 (411S), as well as a warmer and more stratified water column, especially since the 1980s. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction 501S(Da´vila et al., 2002). Moreover, the high sedimentation rates in this area allow for high temporal resolution paleoclimate The Chilean fjord region is an extensive latitudinal area studies. (41–561S) of channels, fjords, and bays that were formed by Several authors have studied the geochemistry of recent glacial erosion during the Quaternary and tectonic sinking of the sediments in Chilean fjords (e.g., Silva and Prego, 2002; Salamanca central Chilean valley (Borgel, 1970). Local remnants of the Last and Jara, 2003; Ahumada and Rudolph, 2004; Pinto and Bonert, Glacial Maximum ice coverage include the North Patagonian 2005; Rojas and Silva, 2005; Sepu´ lveda et al., 2005, this issue; Icefield (46–471S), South Patagonian Icefield (48–521S), and the Mulsow et al., 2009; Silva et al., this issue). Recently, Silva et al. Darwin Mountains Icefield in Tierra del Fuego (54–551S), covering (2009) showed evidence of changes in the sources of the organic a total area of 19,000 km2 (Warren and Aniya, 1999). matter (C/N, d13C) in surface sediments from the Inner Sea of The Chilean fjord region is important for paleoclimate studies Chiloe´ associated with greater contributions of terrigenous matter since it is highly sensitive to variations in the Southern Westerlies, in areas near rivers. In addition, it is important to note that the which are responsible for high precipitation (2000–7000 mm area of the northern fjords (41–441S) is characterized by yrÀ1; Miller, 1976) and maximum river discharges at 421,461, and commercial aquaculture and industrial activities (Otero, 2006; Buschmann et al., 2009). Few sediment studies have focused on the temporal variability (downcore) in assemblages of siliceous microfossils preserved in n Corresponding author at: Instituto de Biologı´a Marina, Facultad de Ciencias, the sediments and the fluctuations in sources of organic matter. Universidad Austral de Chile, Casilla 567, Valdivia, Chile. Tel.: +56 41 220 3557; fax: +56 41 220 7254. At present, the studies that have been carried out in the E-mail address: [email protected] (L. Rebolledo). Puyuhuapi Channel near the Northern Patagonian Icefield 0278-4343/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2010.06.015 L. Rebolledo et al. / Continental Shelf Research 31 (2011) 356–365 357 revealed dramatic changes in the siliceous productivity of the last maximum depth of 200 m. Two main water masses are found in century associated with changes in rainfall, terrigenous contribu- this area: Subantarctic Water and Modified Subantarctic Water tions, and El Nin˜o events (Rebolledo et al., 2005; Sepu´ lveda et al., (influenced by freshwater influxes from rivers). The bathymetric 2005). On a larger temporal scale, paleoceanographic studies constriction in the Gulf of Corcovado (Desertores Pass) impedes carried out in the area of the Jacaf Fjord, Northern Patagonia, have the penetration of Equatorial Subsurface Waters to the north also shown fluctuations in siliceous productivity, sea surface of the pass (Silva et al., 1995). The tide in the Chiloe´ Inner Sea temperature (SST), and precipitation based on geochemical is largely semi-diurnal with variations of up to 6 m (Ca´ceres et al., proxies over the past 2000 years (Rebolledo et al., 2008; 2003). Sepu´ lveda et al., 2009) associated with the Little Ice Age (LIA) In the Inner Sea of Chiloe´ and along the coasts of the fjords, and the Medieval Warm Period (MWP), both widely reported in primary productivity is high, with mean values between 0.5 the Northern Hemisphere and less frequently studied in the and 3 g C mÀ2 dÀ1 (Iriarte et al., 2007), and chlorophyll-a reaches Southern Hemisphere. Furthermore, fluctuations in the San Rafael 0.1–12 mg mÀ3 (Iriarte et al., 2007; Tello and Rodrı´guez-Benito, glacier have also been related to these contrasting climatic 2009). Phytoplankton studies of the water column between periods (Araneda et al., 2007). Puerto Montt and Laguna San Rafael (41–461S) show a predomi- The objective of this research was to study the variability in nance of r-strategist diatoms and small species of the genus siliceous export production preserved in the sediments of the Gulf Thalassiosira and Skeletonema costatum. The greatest phytoplank- of Ancud (421S) over the last century. Based on the content ton cell concentrations are distributed between the Reloncavı´ of siliceous microfossils (diatoms, silicoflagellates, phytoliths, Sound and the Gulf of Corcovado, at the head of Moraleda Chrysophyte cysts) and geochemical proxies (%SiOPAL, organic Channel, and in the Puyuhuapi Channel (Avaria et al., 1997). carbon Corg, nitrogen Ntot, C/N molar, sediment stable isotopes Dissolved inorganic nutrients (orthophosphate and nitrate) in 13 15 d Corg and d N), our record offers evidence of changing the Inner Sea of Chiloe´ are contributed mainly by the oceanic productivity related to Pacific-wide (e.g., El Nin˜o) and local-scale waters, whereas silicic acid is provided by the rivers (Silva et al., events (e.g., rainfall, Puelo River streamflow). 1997). In winter and spring, the study area is characterized by sea surface temperatures of 10–14 1C, salinity ranging from 29 to 33 psu, and inorganic nutrient concentrations of nitrate between 2. Study area 0.5 and 20 mM, silicic acid between 4 and 30 mM, and orthopho- sphate between 0.5 and 3.0 mM(CENDHOC, 2009; Gonza´lez et al., The Inner Sea of Chiloe´ (41–431S; Fig. 1) is connected to the 2010). open ocean on the north through the Chacao Channel and on the The rivers in this area have a pluvio-nival regime, with the south through the Boca del Guafo Passage. This area consists of greatest discharges in the rivers Puelo (678 m3 sÀ1), Yelcho four micro-basins (Reloncavı´ Fjord, Reloncavı´ Sound, Gulf of (363 m3 sÀ1), and Petrohue´ (278 m3 sÀ1), followed by the rivers Ancud, Gulf of Corcovado) interconnected through narrow passes Ren˜ihue (60 m3 sÀ1) and Cochamo´ (20 m3 sÀ1) (Direccio´ n General between islands that give rise to bathymetric constrictions and de Aguas, DGA, 2005). The Comau Fjord (Fig. 1), which is 46 km sills; these contribute to reducing the water flow between micro- long, 1–3 km wide, and has an average depth of 250 m and a basins (Silva et al., 2009), probably leading to long residence time maximum depth of 450 m. Three important rivers (Vodudahue, of the waters. One of the most important of these constriction sills Huinay, Negro) discharge directly into this fjord. Discharges are is the Desertores Pass, which is 5 km wide, 20 m long, and has a usually higher in autumn–winter (April–August) and spring (November–December) and lower in summer to early autumn (January–March). Urrutia et al. (2005) reported a direct relation- ship between precipitation and freshwater discharge, especially in the winter months. 3. Material and methods 3.1. Core collection The core analyzed in this study (MD07-3109H) was obtained during the PACHIDERME cruise in February 2007 on board the R/V Marion Dufresne (MD 159).