Dynamic Holocene Vegetation and North Pacific Hydroclimate Recorded in a Mountain Peatland, Moloka‘i, Hawai‘i

David W Beilman,Jonathan E Nichols,Stephanie Dunbar-Co,Charly Massa,Oliver Elison Timm,Russel Kallstrom

doi:10.3389/feart.2019.00188

Abstract

An organic sediment core from the montane peatland Pēpēʻōpae on the Island of Molokaʻi, Hawaiʻi was studied for multi-proxy evidence of ecological and hydroclimatic change. Following a period of soil development, substantial carbon accumulation and the onset of organic matter stabilization began around 10 ka BP (thousands of years before present) under wet conditions. Continuous but variable peat formation was sustained throughout the Holocene, including maxima in carbon accumulation around 9 and 3 ka, that has resulted in a belowground carbon storage today of 144 kg C m-2. We used sedimentary n-alkane chain length distributions to reconstruct bog vegetation in the context of 14 species of modern bog plant n-alkanes. The peat was not formed by litter inputs from a single plant type or types, but inputs shifted dynamically in their dominance in response to climate. We generated a new chronology for previously-published pollen data from the study site, which showed a similar tempo and response of vegetation change in the forests upwind of the peatland. Hydrogen stable isotope ratios sedimentary C29 n-alkanes show negative departures around 9ka and 3ka consistent with increases in storm-derived rainfall.

Highlights

Knowing the history of hydroclimatic change from a network of sites across the Pacific is important to understanding the external and internal drivers critical to the long-term future of rainfall in the region
While the largest tropical peatlands occur in rainforest lowlands of Southeast Asia (Page et al, 2011), Africa (Dargie et al, 2017), and South American Amazonia (Lähteenoja et al, 2013), low-latitude peatland ecosystems can be found in mountainous terrain, where waterlogged soils are maintained in locally flat areas of heavy rainfall
Dominant pollen types were present throughout the profile (Supplementary Table S3), showing that rainforest vegetation characterized by Metrosideros and Cibotium has been present throughout the Holocene, albeit with substantial shifts in abundance, and including changes in dry-adapted vegetation upwind (Selling, 1948)

Summary

Introduction

Knowing the history of hydroclimatic change from a network of sites across the Pacific is important to understanding the external and internal drivers critical to the long-term future of rainfall in the region Owing to their size, elevation, and isolation, the Hawaiian Islands are the sole location in the central Pacific for comparison to other important freshwater terrestrial proxy records across the Moloka‘i Holocene Hydroclimate region. While the largest tropical peatlands occur in rainforest lowlands of Southeast Asia (Page et al, 2011), Africa (Dargie et al, 2017), and South American Amazonia (Lähteenoja et al, 2013), low-latitude peatland ecosystems can be found in mountainous terrain, where waterlogged soils are maintained in locally flat areas of heavy rainfall Such ecosystems and organic deposits are found, for example, in Papua New Guinea (Hope, 2014), West Kalimantan (Anshari et al, 2004), South American Andes (Benavides et al, 2013) and the Guyana Highlands (Zinck and Huber, 2011) and various Pacific Islands (Rieley and Page, 2015). In addition to holding archives of past hydroclimatic change, mountain peatlands in Hawai‘i provide the opportunity for ecological reconstructions of OM cycling and carbon sequestration relevant to understanding long-term tropical peatland carbon dynamics in general, as well as processes affecting Hawai‘i mountain forests

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