Assessing volcanic influence on clay minerals as weathering proxies during the Paleocene-Eocene Thermal Maximum from Modgunn Hydrothermal Vent (IODP Expedition 396)

Clay mineralogy records provide important climate archives of weathering and hydrology through time, but these paleoclimate signals may be obscured by authigenic or diagenetic overprinting. International Ocean Discovery Program Expedition 396 drilled an expanded Paleocene-Eocene Thermal Maximum (PETM) succession from the Modgunn Vent in the Northeast Atlantic Norwegian Continental Margin. The PETM succession here is marked by frequent occurrence of discrete ash beds (centimetre scale) and by thicker ash-rich deposits. Three major lithological units were identified from the Late Paleocene to the Early Eocene in holes U1568A and U1567B: Late Paleocene bioturbated mudstone (Unit VI), laminated mudstone from the PETM onset and earliest PETM body (Unit V), and ash-rich mudstone in the later PETM body (Unit IV). Smectite is the dominant clay mineral throughout the record, with minor components of illite, kaolinite, and quartz. However, the potential transformation of volcanic ash into authigenic smectite after deposition complicates using clay mineralogy as a proxy for paleoclimate and weathering at this site.

We apply X-ray diffraction (XRD) analyses to quantify the bulk mineralogical composition as well as the clay-sized fraction and electron microscopy (SEM/EDX) to characterise the compositional and morphological changes of the clay-sized fraction. These results enable us to investigate the contribution of volcanism to the clay signal in order to discriminate between continental weathering processes given by clay mineralogy and early diagenesis processes by the input of volcaniclastic material. Morphological analysis of smectites indicate the occurrence of both detrital and authigenic types, but the chemical compositions are clustered by lithological unit rather than type. Detrital smectites in all units are montmorillonite-beidellites, and in Units V and VI authigenic smectites resemble the composition of detrital smectites in the same unit – suggesting a precursory relationship. In Unit IV Mg-rich authigenic smectite (cheto type) makes up >95% of the clay-sized fraction and is associated with enhanced in situ alteration of volcanic ash. This record indicates volcanic ash was relatively well preserved in the latest Paleocene and earliest PETM (Units VI and V) and authigenic smectites were mostly derived from detrital smectite and therefore paleoclimate signals are preserved. In the later PETM, a relative increase in volcanic material to background sedimentation – through increased bioturbation and/or volcanic production – significantly influenced the clay fraction due to the formation of ash-derived authigenic smectite. This process overwhelms the percentage of detrital clay in the XRD record and therefore masked any paleoclimate signals in Unit IV.