An early onset of ENSO influence in the extra-tropics of the southwest Pacific inferred from a 14, 600 year high resolution multi-proxy record from Paddy's Lake, northwest Tasmania

Highlights

• An expansion of non-arboreal pollen between ca. 14.6–13.3 ka at Paddy’s Lake during the Antarctic Cold Reversal.

• Regional hydroclimatic shift at ca. 6.7 ka with ‘modern’ ENSO variability recorded in the tropical Pacific.

• Local-scale shifts in vegetation, fire, and sediment geochemistry synchronous with increases in El Niño activity.

• Possible stationarity in the spatial influence of ENSO over the study region through the Holocene.

Abstract

Tropical El Niño Southern Oscillation (ENSO) is an important influence on natural systems and cultural change across the Pacific Ocean basin. El Niño events result in negative moisture anomalies in the southwest Pacific and are implicated in droughts and catastrophic wildfires across eastern Australia. An amplification of tropical El Niño activity is reported in the east Pacific after ca. 6.7 ka; however, proxy data for ENSO-driven environmental change in Australia suggest an initial influence only after ca. 5 ka. Here, we reconstruct changes in vegetation, fire activity and catchment dynamics (e.g. erosion) over the last 14.6 ka from part of the southwest Pacific in which ENSO is the main control of interannual hydroclimatic variability: Paddy's Lake, in northwest Tasmania (1065 masl), Australia. Our multi-proxy approach includes analyses of charcoal, pollen, geochemistry and radioactive isotopes. Our results reveal a high sensitivity of the local and regional vegetation to climatic change, with an increase of non-arboreal pollen between ca. 14.6–13.3 ka synchronous with the Antarctic Cold Reversal, and a sensitivity of the local vegetation and fire activity to ENSO variability recorded in the tropical east Pacific through the Holocene. We detect local-scale shifts in vegetation, fire and sediment geochemistry at ca. 6.3, 4.8 and 3.4 ka, simultaneous with increases in El Niño activity in the tropical Pacific. Finally, we observe a fire-driven shift in vegetation from a pyrophobic association dominated by rainforest elements to a pyrogenic association dominated by sclerophyllous taxa following a prolonged (>1 ka) phase of tropical ENSO-amplification and a major local fire event at ca. 3.4 ka. Our results reveal the following key insights: (1) that ENSO has been a persistent modulator of southwest Pacific climate and fire activity through the Holocene; (2) that the climate of northwest Tasmania is sensitive to long-term shifts in tropical ENSO variability; and (3) that there has been possible stationarity in the spatial influence of ENSO over this region through the Holocene.

Introduction

The tropical Pacific El Niño-Southern Oscillation (ENSO) is an important component of the global climate system, influencing climatic, physical and ecosystem processes both within and beyond the Pacific Ocean basin. Over the historical period, changes in the intensity and frequency of ENSO events are correlated with a range of important processes; such as, trends in rainfall (Hoerling and Kumar, 2003, Moy et al., 2002, Ropelewski and Halpert, 1987), biomass burning (Fletcher et al., 2015, Kitzberger and Veblen, 2003), mammal population dynamics (Lima et al., 2002, Ogutu and Owen-Smith, 2003) and plant phenology (Asner et al., 2000, Dech and Nosko, 2004, Fletcher, 2015). While longer-term changes in ENSO are implicated in major ecosystem transformations (Fletcher et al., 2014), cultural shifts (Sandweiss et al., 1999, Sandweiss et al., 2001, Turney and Hobbs, 2006, Williams et al., 2008) and both terrestrial (Cao et al., 2004) and marine biogeochemical cycling (Karl et al., 1997). Indeed, the “switching on” of ENSO – the shift from a muted to ‘modern’ ENSO system that occurred in the mid Holocene –is implicated in changes in climate-driven processes across the entire Pacific Ocean basin (Fletcher et al., 2015, Fletcher and Moreno, 2012, Sandweiss et al., 1996, Turney et al., 2004). Despite the widespread recognition of the impact of the mid-Holocene amplification of ENSO, uncertainty exists over both where and when the effects were felt in natural systems (Conroy et al., 2008, Moy et al., 2002). Here, we use a high resolution pollen, charcoal and geochemical analysis of lake sediment from northwest Tasmania, Australia, to assess the influence of millennial-scale ENSO variability over climate variability and consequent landscape dynamics at a site located in the mid-latitudes (extra-tropics) of the southwest Pacific.

The dominant role of hydroclimatic variability over long-term Holocene terrestrial ecosystem dynamics in the mid to high-latitudes of the South Pacific Ocean basin is well described (e.g. Fletcher, 2015, Fletcher et al., 2014, Fletcher et al., 2015, Fletcher and Moreno, 2012, Kilian and Lamy, 2012, Lamy et al., 2010, Mariani and Fletcher, 2016, Moreno, 2004, Pesce and Moreno, 2014, Rees et al., 2015, Stahle et al., 2016); and is understood as a dynamic interaction between zonally symmetric shifts in the extra-tropical southern westerly winds (SWW) and the meridionally asymmetric hydroclimatic signature of the ENSO system (Fletcher and Moreno, 2012, Shulmeister, 1999). In other words, millennial-scale shifts in the SWW during the early to mid-Holocene (ca. 12,000–6,000 years ago – 12-6 ka) drove synchronous in-phase vegetation and fire regime changes across the South Pacific, while the mid to late-Holocene (ca. 6-0 ka) was characterised by sub-millennial scale hydroclimatic oscillations across the region that reflect the influence of long-term phasing of tropical ENSO variability (Fletcher and Moreno, 2012). Critically, while it is clear that ENSO variability approaching ‘modern’ appeared first at ca. 6.7 ka (Moy et al., 2002), the timing of the onset of ENSO inferred from proxy records within the Pacific Ocean basin varies widely (usually between ca. 6-3 ka). In east and southeast Australia, a region in which ENSO variability, in particular the warm (El Niño) phase, is a key determinant of rainfall variability and fire activity (Mariani et al., 2016), a consensus of ca. 5 ka appears to have been reached from a range of proxy types (Donders et al., 2007, Fletcher et al., 2015, Rees et al., 2015, Turney and Hobbs, 2006). An initial onset of the effects of ENSO at ca. 5 ka is consistent with a substantial increase in the frequency of El Niño events recorded in the tropical east Pacific (Moy et al., 2002), but it post-dates the initial amplification of ENSO by ca. 1.7 kyrs, suggesting a spatiotemporal complexity of how ENSO influences the climate of the Pacific Ocean basin.

Tasmania is a cool temperate continental island located in the extra-tropics of the Southern Hemisphere (41–44°S), at the extremity of the ENSO zone of influence in the southwest Pacific and positioned at the climatic interface between the tropical ENSO system and the extra-tropical SWW (Mariani and Fletcher, 2016). Tasmania is, thus, critically located to assess how these two important systems interact in space and time (Fig. 1). Evidence for the influence of ENSO over the climate of Tasmania reveals substantial heterogeneity in where and when the impacts of ENSO were first felt in this complex landscape. Recent studies of lake sediment macroscopic charcoal sequences from southwest Tasmania, a region in which ENSO has little explanatory power over modern rainfall variability (Mariani and Fletcher, 2016), reveal that the prominent spike in El Niño frequency at ca. 5 ka (Moy et al., 2002) was sufficient to teleconnect hydroclimatic anomalies to sites located in western Tasmania that are outside the modern ENSO area of influence (Fletcher et al., 2015, Rees et al., 2015). Initial burning of rainforest vegetation at ca. 6 ka in southern Tasmania, where both ENSO and the SWW are important in governing rainfall anomalies (Hill et al., 2009, Mariani and Fletcher, 2016), is synchronous with a peak in tropical El Niño frequency and reveals an influence of ENSO almost 1 kyrs before the initial ENSO impacts reported in southeast Australia and western Tasmania (Fletcher et al., 2014). Finally, a recent long-term vegetation and fire record from northwest Tasmania on the cusp of the SWW-ENSO zone, reports the first influence of ENSO at ca. 3.5 ka (Stahle et al., 2016), synchronous with a prolonged phase of amplified El Niño activity that is implicated in substantial changes in both natural and cultural systems across the entire Pacific Ocean basin (Sandweiss et al., 1999, Sandweiss et al., 2001, Williams et al., 2008), revealing a substantial lag at that site compared to elsewhere in the region.

Here, we aim to assess the role of tropical ENSO variability in governing fire, vegetation and sediment dynamics over the last 14.6 kyrs in part of western Tasmania where ENSO is currently the dominant driver of inter-annual hydroclimatic variability. Our study site is Paddy's Lake in northwest Tasmania, a high altitude lake (1065 masl), located immediately above the modern treeline. We use a multi-proxy approach that employs analyses of charcoal, pollen, geochemistry and radioactive isotopes to reconstruct fire, vegetation, and sediment dynamics at this site over the past 14.6 kyrs. We then compare our results to regional climate proxy data in an attempt to: (1) assess the long-term climatic framework proposed for this region; (2) determine the timing of the initial impacts of ENSO variability on the local ecosystem; and (3) ascertain the impact of changing tropical ENSO variability on the local ecosystem around Paddy's Lake.