Rock hyrax midden
[5] Hyrax middens contain a diverse range of paleoenvironmental proxies, including fossil pollen and stable carbon, nitrogen and hydrogen isotopes.
At poorly protected sites in arid regions hyrax urine leaves a white, calcium carbonate[11] precipitate on the rocks.
Small overhangs, vertical fractures in cap rocks, and groundwater flow along weakness in the shelter’s architecture may lead to midden degradation if rainfall exceeds a certain amount and/or intensity.
The thickest middens have been found at sites composed of massive, horizontally bedded rock such as granite and quartzites with between ~30 and 480 mm of annual rainfall.
[1] In more humid environments (>800 mm mean annual rainfall), there is little to no evidence of hyraceum accumulation, and middens typically resemble piles of compost, as the masticated plant material in the pellets rapidly decomposes.
[12][13][14] As a result of this work, the vegetation dynamics of this area are some of the best understood for any of the world’s drylands at this timescale, and the critical data provided have dramatically helped define the range of regional climate variability.
These middens are generally reported have no clear stratigraphy, and researchers have thus adopted the methodology of processing them as single samples that provide a palaeoenvironmental snapshot.
[1] Hyrax midden structures and accumulation rates can vary considerably based on the relative proportion of their two primary components, pellets and hyraceum, which is determined by the architecture of the site itself.
Py-GC/MS measurements on samples from two distant sites, Spitzkoppe, Namibia and Truitjes Kraal, Western Cape Province, South Africa produced remarkably similar suites of pyrolysis products,[1] despite their contrasting environmental settings.
Pyrolysis in the presence of a methylating agent tetramethylammonium hydroxide (TMAH) implied that benzamide is a monomer of a larger polymeric structure, the major organic component of the hyraceum OM.
[32] This is further supported by the ubiquity of benzamide within solvent extracts and it is probable that it is derived from hippuric or benzoic acid, which are common metabolites in ruminants.
There are also clear similarities with the spectrum of benzamide, particularly at Spitzkoppe, which is consistent with the pyrolysis data[1] Part of the extraordinary potential of hyrax middens as palaeoenvironmental archives is the large range of proxies that are contained within them.
This is valuable when comparing proxies that reflect vegetation change (e.g. fossil pollen) and those that are primarily influenced by climate (e.g. δ15N), as the relative roles of climatic forcing versus vegetation dynamics related to competitive processes within an ecosystem can be better resolved, resulting in a fuller and more reliable understanding of palaeoenvironmental dynamics[3][7] Hyrax middens contain well-preserved micro plant material including pollen, which is sealed in middens by hyraceum, protecting it from microbial activity and decay.
The airborne pollen rain is incorporated by (1) collecting on the surface of the midden, (2) being brought in on the fur of the hyraxes, or (3) being ingested as dust on dietary items such as plant leaves or drinking water.
[40][42][1] The dietary component may also represent the ingestion of flowers, which may result in the occasional over-representation of pollen of certain plant species in the pellet fraction of certain middens.
There are some potential drawbacks for the palynological analysis of middens, however, as the diverse taphonomic vectors can complicate interpretations if they are not adequately considered and controlled for.
[56][54] A number of options might explain this, but it is assumed that as any given pellet represents what was eaten in the last day(s), there will be substantial inter-seasonal and inter-annual variation in the pollen preserved.
[50] Structured studies to clarify the relative influence of regional (aeolian) and local (fur) signals in the pollen preserved in hyraceum remain to be completed.
At least in some cases, aeolian inputs appear to be negligible as some middens that have accumulated in vertical cracks - precluding the incorporation of pellets and direct contact with the animals - have been found to be devoid of pollen.
[58] This is useful in climatic transition zones, such as the Western Cape Province of South Africa, where modern rainfall seasonality has a strong impact on C3/C4 grass distributions.
As such, other studies have focussed on the use of δ15N data as a potential proxy for water availability in the environment[6][7][4][5] In palaeoclimatology, the variables for which reconstructions are most often sought are humidity and temperature.
In contrast to the initial findings of Heaton et al.,[68] subsequent studies of soils and plants across aridity gradients, indicate a clear negative correlation between 15N and rainfall.
Soil moisture and δ15N also vary significantly over short, sub-seasonal timescales[76] and, combined, these fine-scale spatio-temporal variations need to be adequately controlled for if reliable δ15N-climate correlations are to be identified.
To extend the findings of Murphy and Bowman[80][81] to the study of excrement and hyrax middens, one can consider the studies of (1) Codron and Codron,[75] which concluded that faecal δ15N correspond to changes in plant δ15N, and (2) Sponheimer et al.,[82] which found that, while preferential urinary excretion of isotopically light nitrogen may occur under conditions of disequilibrium, an unstressed animal at “steady state” will have equivalent dietary and excreta δ15N.
