Speleothem master chronologies: combined Holocene 18O and 13C records from the North Island of New Zealand and their palaeoenvironmental interpretation

Abstract

The stable isotope records of four stalagmites dated by 19 TIMS uranium series ages are combined to produce master chronologies for δ 18O and δ 13C. The δ 18O records display good overall coherence, but considerable variation in detail. Variability in the δ 13C records is greater, but general trends can still be dis cerned. This implies that too fine an interpretation of the structure of individual isotopic records can be unreliable. Speleothem δ 18O values are demonstrated to show a positive relationship with temperature by comparing trends with other proxy records, but also to respond negatively to rainfall amount. Speleothem δ 13C is con sidered to be most influenced by rainfall. The postglacial thermal optimum occurred around 10.8 ka BP, which is similar in timing to Antarctica but up to 2000 years earlier than most Northern Hemisphere sites. Increasingly negative δ 18O values after 7.5 ka BP indicate that temperatures declined to a late mid-Holocene minimum centred around 3 ka BP, but more positive values followed to mark a warm peak about 750 years ago which coincided with the ‘Mediaeval Warm Period’ of Europe. Low δ 18O values at 325 years BP suggest cooling coincident with the ‘Little Ice Age’. A marked feature of the δ13C record is an asymmetric periodicity averaging c. 2250 years and amplitude of c. 1.9‰. It is concluded that this is mainly driven by waterbalance variations with negative swings representing particularly wet intervals. The δ18O record shows a higher-frequency cyclicity with a period of c. 500 years and an amplitude of c. 0.25‰. This is most likely to be temperature-driven, but some swings may have been amplified by precipitation.

Modification and preservation of environmental signals in speleothems

“…Several published examples exist demonstrating an absence of significant isotope shifts in slow drips through at least a hydrological year (Yonge et al, 1995;Caballero et al, 1996;Williams et al, 2004). This is consistent with a dominance of seepage-fed flow, where the seepage reservoir and/or the overlying soil and epikarst has a structure that permits thorough mixing.…”

“…In contrast, cave dripwaters in Barbados are closer to the lighter end of the monthly rainfall values because of limited recharge in drier months when the isotopically heavier precipitation occurs (Jones et al, 2000;Mickler et al, 2004); this is the pattern expected also in arid climates (Gat, 1996). In temperate climates little deviation from mean compositions of precipitation is usual (Yonge et al, 1985;Williams et al, 2004).…”

Speleothems as indicators of wet and dry periods

“…Oxygen isotope data are now being generated on a sub-annual scale (Treble et al, 2005c) which allows summer and winter precipitation to be distinguished. Although many drips have homogenized compositions (Williams et al, 2004), the presence of a fracture-fed component allows the seasonal variation to be distinguished (Fairchild et al, 2006b). High-resolution trace element data can be generated by a variety of techniques and annual variations are normally found.…”

Medieval Warming, Little Ice Age, and European impact on the environment during the last millennium in the lower Hudson Valley, New York, USA

“…Maintenance of fields and grasslands for berries and wild game prior to European settlement is a possible cause for increased burning (Williams, 1989), but large wildfires started by Native Americans are considered unlikely (Day, 1953;Russell, 1983). Natural disturbance or land clearance and maintenance by Native Americans in the large river valleys of northeastern North America (Day, 1953;Williams, 1989) would allow lightloving species such as Ambrosia to sporadically thrive prior to European settlement.…”

“…Native Americans often settled in river valleys, and those living in southern New England subsisted more on agricultural practices than from hunting alone (Cronon, 1983;Williams, 1989). Maintenance of fields and grasslands for berries and wild game prior to European settlement is a possible cause for increased burning (Williams, 1989), but large wildfires started by Native Americans are considered unlikely (Day, 1953;Russell, 1983).…”