Lake Waiau, July 2004

Paleoclimate records from tropical regions, like Hawai'i, are essential to understanding past changes in Earth's climate system, equator-pole climate linkages and the sensitivity of tropical regions to future climate change, yet few continuous climate records extending to the last deglaciation are available. Hawaii's isolated position in the Pacific provides an important site for reconstructing past tropical response to global climate change. The Holocene, which encompasses the past 10 kyr of Earth history, has experienced significant climate changes over various time scales. El Niño-Southern Oscillation (ENSO) events and variations in the Aleutian low pressure system, represented by the North Pacific Index (NPI), have influenced the climate and particularly the hydrologic cycle in the Pacific basin [Kitoh and Murakami, 2002; Minobe and Nakanowatari, 2002]. Some climatic intervals of relative cold or warmth have been identified at high latitudes in the northern hemisphere, but the low latitude response to these intervals is still not well known. Examination of the effects of those cold and warm periods on ENSO activity and on the Aleutian low variations will provide information on climate dynamics and on the possible effects of global warming on the hydrological cycle of the Pacific basin. In particular, a record of the effects of global climate phenomena such as the "Little Ice Age" (1600-1800 yr AD), the "Medieval Warm Period" (900-1100 yr AD) and the Mid-Holocene high stand (around 5000 cal yr BP) on low latitude climates would be of great importance in understanding equator-pole teleconnections. Such a record could potentially be extracted from Lake Waiau sediments

Lake Waiau setting

Lake Waiau lies in the crater of Pu'u Waiau cinder cone on Mauna Kea volcano at an elevation of 3970m above sea level. It has a small average surface area (~0.6 ha) and its depth varies between 0.5 and 2.3m. When the lake depth exceeds 2.3m, water is lost by spillover into a low area on the west side of the lake [Ehlmann et al., 2004; Woodcock, 1980]. The area surrounding the lake and the alpine zone of Mauna Kea (from 2896m to the summit at 4200m) are a botanical desert [Massey, 1978].

Pu'u Waiau was dated at 107,000 ± 13,000 years B.P. using K-Ar in a lava flow [Wolfe, 1997]. Geologic evidence indicates that the lake was probably formed in Pleistocene time when Mauna Kea was last glaciated. Indeed, during the last glacial maximum, the ice cap on Mauna Kea covered about 70 km2 and terminated as low as 3200m asl [Porter, 1979]. Porter et al., [1977] identified the last eruption of the Mauna Kea at 4500 years B.P. This was confirmed by the age of the first coarse ash layer in a sediment core of lake Waiau [Woodcock, 1966]. Lava flows indicate that Mauna Kea certainly erupted also at 9080 and 22,150 years B.P. [Porter et al., 1977].

Lake Waiau hydrology

Woodcock [1980] showed that Lake Waiau water level variations reflect the average amount of precipitation over the Hawaiian islands. During dry years, the lake can drop to a level of around 0.5m. During wet years, the lake can overflow and will have a temporal average depth of 1.2m. Recent work by Ehlmann et al. [2004] shows that the oxygen isotopic composition (δ ¹⁸O) and the deuterium composition (δ D) of the lake water follow the precipitation-evaporation balance over the lake. δ ¹⁸O and δ D decrease because of precipitation and increase because of evaporation. If precipitation is very low during one or a few consecutive years, the lake water will become very isotopically heavy, so that the signature of an unusual dry event will be preserved during more than one year.

Lake Waiau fauna and sedimentary record

The lake flora consist of a few species of planktonic diatoms and algae and an epilithic mat of primarily amorphous blue-green algae.

Lake Waiau has been a natural trap for sedimentary material since its formation. At the bottom of this lake lie more than 7.5m of sediments [Woodcock, 1966], which are distinctly laminated and composed mainly of lacustrine silt, clay and volcanic ash.

Peng and King [1992] took a 6m long core in Lake Waiau (the longest core taken), which spanned the last 14,000 years. It is estimated that the sediment thickness of the lake is about 8m, which according to their age model, should span the last 19,000 years.

Variations of lake fauna

It is important for any paleoclimate study depending on phytoplankton to understand the intra as well as interannual variations in the lake fauna. Unfortunately, only a few observations are available for lake Waiau.

Date	June, 1st 2000	July, 24th 2004	October, 30th 2004
Water column: Dominant species, minor species	Chlorophytes (Scenesdesmus), Diatoms, Chlorophytes (Cosmarium)	Cyanobacteria (Anabaena), Chlorophytes (Cosmarium)	Cyanobacteria (Anabaena), Chlorophytes (Cosmarium)
Surface sediments: Dominant species, minor species	Chlorophytes (Scenesdesmus), Diatoms	Cyanobacteria (Anabaena), Cyanobacteria (Oscillatoria), Diatoms	Cyanobacteria (Anabaena), Cyanobacteria (Oscillatoria), Diatoms

Extraction and identification of diatoms from Lake Waiau sediment core

Diatoms from a sediment core taken by A. Woodcock in 1966 in Lake Waiau have been isolated from the sediments following a method adapted from Morley et al. [2004]. The species of the diatoms isolated have been identified using SEM analysis. 5 species have been identified: Hantzschia amphoxys, Nitzschia frustulum, Nitzschia vitrea, Pinnularia abaujensis and Navicula radiosa. 4 of them (Hantzschia amphoxys, Nitzschia frustulum, Nitzschia vitrea and Navicula radiosa) have been also isolated from surface sediments of the lake. The diatoms isolated don't present any sign of alteration. Oxygen isotopic analyses could eventually be performed on the diatom frustules isolated from the lake sediments.

Climate variations in Hawai'i

References