Black layers in speleothems as a proxy for natural and anthropic processes

ABSTRACT

Black layers (BL) in speleothems record natural and anthropogenic processes that complement calcite laminae information. However, their complex composition and genesis often remain understudied. This work presents a multi‐analytical characterization (petrography, dating, trace‐element geochemistry, SEM‐EDS, TEM and computed axial tomography) of BL observed in four Holocene stalagmites from Atapuerca (northern Spain). Based on petrographic criteria (erosional features, alteration of underlying calcite and crystal fabric changes), two BL categories are defined: erosive (BL‐E1 and BL‐E2) and non‐erosive (BL‐C1 and BL‐C2). BL‐E1 shows dissolution surfaces, neomorphism (micritization/sparitization) and enrichments in lithophile and biophile elements; the subtype BL‐E1b includes phosphatic precipitates suggesting bat guano input. Their formation reflects arid intervals with reduced growth rates and/or the occurrence of growth hiatuses, followed by infiltration of terrigenous material and organic matter. BL‐E2 lacks allochthonous minerals and shows only subtle geochemical anomalies, being linked to humid periods where undersaturated drip waters caused erosion without significant detrital input. BL‐C1 consists of calcite bands stained by organic colloids with no geochemical enrichment, indicating moderate humidity. BL‐C2 is composed of soot particles and charcoal fragments, enriched in biophile elements and is attributed to anthropogenic fires inside the cave coinciding with intense human occupation phases (Neolithic to Bronze Age). This study demonstrates that BL in speleothems record both natural and anthropogenic processes and highlights the importance of their study for palaeoenvironmental reconstructions.