Seasonal variability in turbidity currents in Lake Ohau, New Zealand, and their influence on sedimentation

Layers of sediment that are deposited on the floor of Lake Ohau, New Zealand, offer a means to reconstruct past climate conditions in the Southern Hemisphere at subdecadal and annual resolution. A robust understanding of the modern physical processes that control the influx and dispersal of sediment in the lake is required to reconstruct climate from these sedimentary archives. In this study, water temperature and velocity measurements collected during 2012–13 were analysed to determine the primary physical processes that influence sediment transport in the lake. Sediment input from river inflow occurs throughout the year but exhibits strong seasonal variation. Large inflow events (Q. 500 m s ) that follow strong summer rainstorms trigger high-concentration turbidity currents, which are themain agents for sediment delivery and deposition. During winter, smaller turbidity currents also occur after rain events and contribute to annual sediment accumulation. In addition, large internal waves were observed during the summer and may influence sedimentation. In conclusion, several processes including river inflow, internal waves and convectively driven flows control sediment deposition and accumulation in the Lake Ohau system. We utilise these observations to establish a conceptual model to explain the observed infill stratigraphy in Lake Ohau and guide interpretation of the longer sedimentary record. Received 3 February 2015, accepted 7 August 2015, published online 4 November 2015 Introduction Sedimentation patterns in alpine and temperate lakes can be strongly influenced by the magnitude and physical properties of river inflows (Hamblin and Carmack 1978; Alavian et al. 1992; De Cesare et al. 2006). In particular, increased river inflow in response to rainfall and the spring freshet can form gravity currents carrying large sediment loads into lakes (Weirich 1986; Desloges and Gilbert 1994; Gilbert et al. 2006; Amann et al. 2014). Seasonal variability in river-fed lakes can influence sedimentation and produce clastic varves, e.g. relatively thick layers of coarse sediment during summer and thinner layers of fine sediments during winter (e.g. Leemann and Niessen 1994; Gilbert and Butler 2004). In order to use clastic varves to reconstruct past climate, establishing the linkages between meteorological, limnological and sedimentary processes in lakes is required (e.g. Leemann and Niessen 1994; Hardy 1996). Lake-monitoring programs provide measurements of physical properties in the water column (e.g. temperature, river inflow, velocities) and sediment characteristics (e.g. turbidity, particle concentration, particle size) that can help identify the control processes on sedimentation and to accurately interpret the sedimentary record preserved in the lake bed (Leemann and Niessen 1994; Tylmann et al. 2012; Ojala et al. 2012). Many lake systems and their associated sedimentation regimes have been studied in the Northern Hemisphere (e.g. Pharo and Carmack 1979; Mulder and Syvitski 1995; Gilbert and Crookshanks 2009) and have been used to generate annually resolved palaeoclimate reconstructions (e.g. Gilbert 1975; Desloges 1994; Leemann and Niessen 1994; Hardy 1996; Gilbert et al. 2006; Amann et al. 2014; and many others). Similar studies are largely absent from the SouthernHemisphere (Ojala et al. 2012) despite the potential to gain insight into Pacific and Southern Hemisphere climate modes including El Niño–Southern Oscillation and the Southern Annular Mode, which regulates the Southern Hemisphere westerly winds (Thompson and Solomon 2002; Ummenhofer et al. 2009). Located on the South Island of New Zealand, Lake Ohau offers a prime target to address this gap in global climate records as it sits near the northern boundary of the westerly winds that dominate atmospheric circulation. A 70-m-thick sequence of CSIRO PUBLISHING Marine and Freshwater Research http://dx.doi.org/10.1071/MF15043 Journal compilation CSIRO 2015 www.publish.csiro.au/journals/mfr sediments in the lake basin offers the potential to examine climate evolution in the Southern Hemisphere since the end of the last glacial maximum (c. 17 000 years ago). Fine-scale (mm) laminations preserved in lake floor sediment likely form in response to seasonal variability in hydroclimate (Roop et al. 2015). However, detailed examination of hydrodynamic conditions through the seasonal variations is required to improve our understanding of sediment dispersal within the lake and identify the cause(s) of millimetre-scale varves. In this paper we examine temperature, current velocity, and turbidity data acquired near the dominant inflow of Lake Ohau to explore the effect of changing physical properties in the water column on sedimentation over an annual cycle. Constraining the role of large flood events is a particular focus as heavy rainstorms can trigger sedimentation rates on the order of 15 mm year 1 at the distal end of the lake and.200mmyear 1 near the river delta (Roop et al. 2015). Clearly, flood-related large turbidity currents, internal waves and convectively driven density currents have an effect on long-term sedimentation patterns in the Lake Ohau system. The development of a conceptual model that integrates these elements is a primary aim of this research. Material and methods Site description Lake Ohau (44814.0400S, 169851.2400E; 520 m above sea level) is one of three north–south trending lakes formed during the rapid glacial retreat between 17 900 and 17 4000 years ago following the last glacial maximum on the South Island of New Zealand (Fig. 1b) (Putnam et al. 2013). The lake lies in the intermontaneMackenzie Basin, is 18.5 km long, up to 5 kmwide 400 000 0.00 E 410 000 0.00 E 420 000 0.00 E 5 10 0 00 0 0. 00 N 5 11 0 00 0 0. 00 N 5 12 0 00 0 0. 00 N