Scale considerations in using diatoms as indicators of sea‐level change: lessons from Alaska

Abstract

This paper assesses variations in quantitative reconstructions of late Holocene relative sea‐level (RSL) change arising from using modern diatom datasets from different spatial scales, applied to case studies from Alaska. We investigate the implications of model choice in transfer functions using local‐, sub‐regional‐ and regional‐scale modern training sets, and produce recommendations on the creation and selection of modern datasets for reconstructing RSL change over Holocene timescales in tidal marsh environments comparable with those in Alaska. We show that regional modern training sets perform best in terms of providing fossil samples with good modern analogues, and in producing reconstructions that most closely match observations, where these are available. Local training sets are frequently insufficient to provide fossil samples with good modern analogues and may over‐estimate the precision of RSL reconstructions. This is particularly apparent when reconstructing RSL change for periods beyond the last century. For reconstructing RSL change we recommend using regional modern training sets enhanced by local samples. Copyright © 2012 John Wiley & Sons, Ltd.

Detection limits of tidal-wetland sequences to identify variable rupture modes of megathrust earthquakes

“…The addition of further samples to the modern training set available for transfer function methods, following each field season since the original study (Hamilton and Shennan, 2005a), raises a methodological debate regarding transfer function methods that we raised previously Watcham et al, 2013). A modern dataset dominated by modern samples from the local site,…”

“…We assess goodness of fit between each fossil sample and the modern dataset with a dissimilarity coefficient, using the 20th percentile of the dissimilarity values for the modern samples as the cut-off between 'close' and 'poor' modern analogues for fossil samples, and the 5th percentile as the threshold for defining 'good' modern analogues. For reconstruction of the elevation at which the fossil sediment accumulated, termed paleo-marsh surface elevation, PMSE, we present sample-specific 95.4% (2σ) error terms.The addition of further samples to the modern training set available for transfer function methods, following each field season since the original study (Hamilton and Shennan, 2005a), raises a methodological debate regarding transfer function methods that we raised previously Watcham et al, 2013). A modern dataset dominated by modern samples from the local site,…”

Salt marshes as late Holocene tide gauges

“…When this occurs we must develop a training set that includes modern samples from other marshes that provide analogues for these past environments. These datasets are often called 'regional' though the definition can vary from author to author, with regional datasets varying from an estuary to a 100 km stretch of coastline to a whole country Gehrels et al, 2001;Kemp et al, 2009b;Leorri et al, 2008;Szkornik et al, 2006;Watcham et al, 2013;Zong and Horton, 1999). There is on-going debate as to the benefit of one approach over another (e.g.…”

Salt-marsh reconstructions of relative sea-level change in the North Atlantic during the last 2000 years

“…We apply the modern 259 analogue technique (MAT) to assess the latter by quantifying the similarity between each fossil 260 sample and the modern training set (Birks, 1995). We use the 20 th percentile of the minimum 261 dissimilarity coefficients (MinDC) calculated between all modern samples as the cut-off between 262 'good' and 'poor' modern analogues (Watcham et al, 2013). These thresholds are used for visual 263 guidance only.…”