Material biológico

In contrast to the stability of the early to mid Holocene (~ 10,000–5000 cal. yr BP), the mid to late Holocene (~ 5000–2000 cal. yr BP) involved ENSO intensification from ~ 7000 cal. yr BP (Moy et al., 2002) and the onset of active ENSO cyclicity (Donders et al., 2008). In Northland this period is characterised by an extended period of region-wide floodplain alluviation (Fig. 7.4) and floodplain terrace development (cutting and refilling of the valley floor) in partly confined valley floor settings in Northland (Richardson et al., in review-b). Three episodes of river activity have been identified in the Northland fluvial record, the most significant phase occurring between 3500 and 2800 cal. yr BP.

The majority of Northland palaeovegetation records detect some degree of climate change during the mid to late Holocene (Dodson et al., 1988; Kershaw and Strickland, 1988; Elliot et al., 1995; Striewski et al., 1996; Elliot et al., 1997; Elliot, 1998; Elliot et al., 1998; Elliot et al., 2005). Pollen evidence from swamp deposits in northwest Northland indicate increased seasonality, summer drought, wetter cooler winters and increased cyclonic activity, with declines in frost intolerant taxa and increases in the abundance of hardy podocarps between ~ 5000 and 2500 cal. yr BP (Fig. 7.4) (Elliot, 1998; Elliot et al., 2005). Pollen records from northern Northland show evidence of forest disturbance and seral trends after ~ 4000 cal. yr BP (Striewski et al., 1996), and in eastern Northland, climate after ~ 4000 cal. yr BP was considered cooler and drier with more intense cyclonic activity before climate

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amelioration from ~ 2000 to 1600 cal. yr BP (Elliot et al., 1997). In eastern Northland palaeovegetation records from two sites indicate major change around ~ 3500 cal. yr BP, with signs of repeated forest disturbance and erosion, summer drought and cooling (Kershaw and Strickland, 1988; Elliot et al., 1998). Increase in the frequency of drought conditions was suggested as the cause of a decline in kauri in far northern New Zealand after ~ 3000 cal. yr BP (Dodson et al., 1988) and swamp deposits in western Northland show a pattern of kauri expansion and decline due to fluctuation in substrate wetness after ~ 3500 cal. yr BP (D'Costa et al., 2009). Other major changes detected in Northland records that correlate with the major phase of river activity (3500–2800 cal. yr BP) include increases in effective precipitation detected in swamp deposits near Whangarei, at 3400 cal. yr BP (Kershaw and Strickland, 1988) and around 3000–2000 years ago (Newnham, 1992).

The palaeoclimate proxy records from the Northland region suggest that the episode of river activity at 3500–2800 cal. yr BP was a climatically forced response to ENSO intensification, with evidence for increased seasonality and disturbance in the vegetation record around this time (Fig. 7.4). In addition, the phase relationship between SAM and ENSO may have also been playing a role in influencing river activity and amplifying ENSO teleconnections between the tropical Pacific and higher Southern Hemisphere latitudes. The cluster of fluvial

14_{C dates and a peak in relative probability in the Northland river activity record between}

3500 and 2800 cal. yr BP coincided with the general trend of a more disturbed and cooler mid to late Holocene climate regime, increases in El Niño frequency after ~ 3200–2800 yr BP (Sandweiss et al., 2001) and increased impacts in teleconnected regions from around 3000 cal. yr BP (Donders et al., 2008).

Storm records reconstructed from lake sediments in Ecuador and eastern North Island, New Zealand, indicate that between ~ 5000 and 2000 cal. yr BP storm frequencies were in phase (Gomez et al., 2011). This occurred at a time when tropical Pacific sea surface temperatures and central Antarctic surface temperatures signalled connectivity between the low and high latitudes and correlation between the SAM and ENSO climate modes (Gomez et al., 2011). The episode of Northland river activity coincides with a synchronised peak in storm activity at Laguna Pallcacocha and Lake Tutira (from 3300 to 3100 cal. yr BP), under La Niña-like conditions and during the positive phase of the SAM (Gomez et al., 2011). This reinforces the assertion that river activity in northern New Zealand is associated with a predominance of moisture bearing northeasterly atmospheric flow under a blocking synoptic regime (promoted by La Niña phases of the Southern Oscillation and positive phases of the SAM) (Richardson et al., 2013).

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Fig. 7.4. Schematic of regional palaeoclimate, palaeovegetation and Northland river

behaviour for the Holocene. (a) Relative CPF plot of Holocene river activity in Northland, New Zealand, based on analysis of 14_{C-dated Holocene fluvial units. Vertical grey bars}

highlight episodes of river activity and vertical dark grey bars highlight the most significant episodes. (b) Summary of Northland Holocene floodplain development (Richardson et al., in review-b). (c) Northland palaeovegetation zones. (d) Temperature and precipitation inferred from composite curves of speleothem į18O and į13C for the Waitomo district of central-west North Island (Williams et al., 2010).

Circulation changes associated with the major climate modes influence rainfall patterns and the incidence of cyclonic activity in New Zealand, which in turn have an impact on

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vegetation and sediment flux, and ultimately river system dynamics. It is most likely that the 3500–2800 cal. yr BP phase of river activity in the Northland record was a response to climatically driven catchment perturbation with potential amplification due to the unique nature of the Northland vegetation. A major component of the Holocene pre-settlement vegetation in far northern New Zealand was Agathis australis (kauri), the long-lived (> 600

years) emergent canopy tree which has a preference for a warm and moist climate (Ogden et al., 1992). Stand regeneration of kauri is episodic in nature and depends on large-scale disturbance (Ogden et al., 1992), creating a vulnerability toward cohort collapse if growth conditions deteriorate or through storm wind throw, potentially leading to large-scale catchment destabilisation.

Despite the fact that New Zealand has a high degree of regional climate variability (Ummenhofer and England, 2007), alternative palaeoclimate proxies and river activity records from other regions of New Zealand suggest that the climatic perturbation detected in the Northland river activity record at 3500–2800 cal. yr BP was not restricted to far northern New Zealand. Stable isotope speleothem data from Waitomo (Figs. 7.1 and 7.4 in central- west North Island (CWNI) provide a proxy record of Holocene palaeoprecipitation and palaeotemperature for the northern New Zealand coherent precipitation region (Williams et al., 2010). Northland river activity at 3500–2800 cal. yr BP corresponds to a remarkably cool phase (3400–2900 cal. yr BP) at the culmination of a gradual decline in temperature from the mid Holocene (Fig. 7.4). Speleothem records from northwestern South Island (NWSI) also a show negative excursion in į18_{O values concurrent with a glacier advance}

(mean moraine age of ~ 3200 years ago) in the Southern Alps (Schaefer et al., 2009; Williams et al., 2010). Peaks in į13_{C values at 3400 yr BP in both the CWNI and NWSI}

records suggest that the beginning of the phase of river activity in Northland occurred when these regions were dry as well as cool, although the CWNI speleothem data show considerable variation in precipitation during this time (Williams et al., 2010).

There is also evidence that this late Holocene cool event had a widespread impact on river systems in other regions of New Zealand, with river activity identified in five out of the six homogenous precipitation regions at ~ 3200 cal. yr BP (Richardson et al., 2013). Probability-based records of river activity for northern North Island, southwestern North Island, eastern North Island, northern South Island and eastern South Island (Richardson et al., 2013) show that in these climate regions rivers were also undergoing phases of river activity involving floodplain sedimentation at the same time as Northland systems. The only region that did not experience a significant phase of river activity during this time was the western and southern South Island climate region. Probability-based records of Holocene

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river activity suggest that in the South Island increased river activity is a response to enhanced westerly atmospheric circulation associated with a predominance of trough regime synoptic type (negative SAM-like circulation) (Richardson et al., 2013). In the North Island episodes of river activity are driven by increased meridional atmospheric circulation associated with blocking regime synoptic conditions (La Niña-like and positive SAM-like circulation) (Richardson et al., 2013). The extent of river activity in New Zealand centred on the period ~ 3400–3000 cal. yr BP overrode a strong tendency toward an out-of-phase relationship between river activity in northern and southern regions (Richardson et al., 2013) and suggests that climate at the time was being more influenced by subtropical atmospheric influences.

Marked alteration in fluvial processes around the mid Holocene was also detected in eastern North Island floodplain, continental-shelf and continental-slope sediment records (Gomez et al., 2004). These sediment records preserve a textural change at ~ 4000 cal. yr BP, representing a shift in the sediment production process, from fluvial incision to a predominance of landslide derived material as a result of increased storms (Gomez et al., 2004). This mid Holocene climate change signal was interpreted as an intensification of atmospheric circulation and ENSO activity in the region (Gomez et al., 2004).

The episode of river activity detected in New Zealand and Northland records between 3500 and 2800 cal. yr BP also correlates with the well documented late Holocene cold event, one of six prominent cold periods, recognised in a number of palaeo-environmental records from around the globe (Wanner et al., 2011). The mechanism behind these Holocene cold relapses is thought to involve decreases in Boreal summer insolation and other processes, including volcanic eruptions, solar activity, melt-water flux and thermohaline circulation (Wanner et al., 2011). The Holocene cold event from 3300 to 2500 cal. yr BP (Wanner et al., 2011) and dry period (centred on ~ 3100 cal. yr BP) occurred during a time when summer insolation was increasing in the Southern hemisphere above that of the Northern Hemisphere (from ~ 4200 cal. yr BP to present). The cold relapse at 3300 to 2500 cal. yr BP exhibits the closest correlation with a Bond cycle, event 2 with a peak at ~ 3000 yr BP (Bond et al., 2001). Mayewski et al. (2004) also identified a global-scale rapid climate change event at 3500– 2500 cal. yr BP, characterised by a pattern of cooler poles and tropical aridity, most likely forced by solar variability and changes in orbital insolation. This suggests that the Holocene event detected in the New Zealand river activity record and the Northland record between 3500 and 2800 cal. yr BP was globally extensive. It is therefore reasonable to assert that the phase of river activity and catchment destabilisation in Northland occurred in response to a solar forced global cold event during a period of rapid climate change.

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