Layer-bounding surfaces in stalagmites as keys to better paleoclimatological histories and chronologies

Abstract

Petrographic recognition of layer-bounding surfaces in stalagmites offers an important tool in constructing paleoclimate records. Previous petrographic efforts have examined thickness of layers (a possible proxy for annual rainfall) and alternation of layers in couplets (a possible indicator of seasonality). Layer-bounding surfaces, in contrast, delimit series of layers and represent periods of non-deposition, either because of exceptionally wet or exceptionally dry conditions.Two types of layer-bounding surfaces can be recognized according to explicitly defined petrographic criteria. Type E layer-bounding surfaces are surfaces at which layers have been truncated or eroded at the crest of a stalagmite. Keys to their recognition include irregular termination of layers otherwise present on the stalagmite’s flank, dissolutional cavities, and coatings of non-carbonate detrital materials. Type E surfaces are interpreted to represent wet periods during which drip water became so undersaturated as to dissolve pre-existing stalagmite layers, and thus they necessarily represent hiatuses in the stalagmite record. Type L layer-bounding surfaces are surfaces below which layers become thinner upward and/or layers have lesser lateral extent upward, so that the stalagmite’s layer-specific width decreases. They are thus surfaces of lessened deposition and are interpreted to represent drier conditions in which drip rate slowed so much that little deposition occurred. A Type L surface may, but does not necessarily, represent a hiatus in deposition. However, radiometric age data show that Type L surfaces commonly represent significant hiatuses.These surfaces are significant to paleoclimate research both for their implications regarding climate change (exceptionally wet or dry conditions) and in construction of chronologies in which other data, such as stable isotope ratios, are placed. With regard to climate change, recognition of these surfaces provides paleoclimatological information that can complement or even substitute for geochemical proxies. With regard to chronologies, recognition of layer- bounding surfaces allows correct placement of hiatuses in chronologies and thus correct placement of geochemical data in time series. Attention to changing thickness of annual layers and thus to accumulation rate can also refine a chronology. A chronology constructed with attention to layer-bounding surfaces and to changing layer thickness is much more accurate than a chronology in which hiatuses are not recognized at such surfaces

Three distinct Holocene intervals of stalagmite deposition and nondeposition revealed in NW Madagascar, and their paleoclimate implications

“…This mineralogical trend is not surprising as calcite commonly forms under wetter conditions (e.g., Murray, 1954;Pobeguin, 1965;Siegel, 1965;Thrailkill, 1971;Cabrol and Coudray, 1982;Railsback et al, 1994;Frisia et al, 2002). Additionally, non-carbonate detrital materials are commonly abundant with varying grain size (i.e., from silt to sand size; Railsback et al, 2013).…”

“…3). Physical properties of stalagmites that are evidence of extreme dry and wet events are summarized in Table 1 of Railsback et al (2013) and the mechanisms are explained in their Fig. 5.…”

“…Decreases in layer thickness and stalagmite width upward are indications of lessening deposition (thus the name "L"; Railsback et al, 2013). Aragonite is a very common mineralogy below a Type L surface, especially in warmer settings.…”

“…Major hiatuses in stalagmite deposition could be marked by a variety of features, including the presence of erosional surfaces, chalkification, dirt bands and/or detrital layers, offsetting of the growth axis, and/or sometimes by color changes (e.g., Holmgren et al, 1995;Dutton et al, 2009;Railsback et al, 2013Voarintsoa et al, 2017a). Railsback et al (2013) were able to identify significant features in stalagmites that allow distinction between nondeposition during extremely wet conditions (Type E surfaces) and non- deposition during extremely dry conditions (Type L surfaces; Fig. 3).…”

“…The StalAge scripts were run on the statistics program R version 3.2.2. The age models were adjusted considering hiatal surfaces identified in the samples, using the approach of Railsback et al (2013;see their Fig. 9).…”