Dorothea S. Macholdt

• Dr.
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X-ray microspectroscopy is a powerful analytical method in geoscientific and environmental research as it provides a unique combination of nanoscale imaging with high spectroscopic sensitivity at relatively low beam-related sample damage. In this study, “classical” scanning transmission soft X-ray microscopy (STXM) with X-ray absorption spectroscop...

Rock varnishes are μm-thin, dark, manganese(Mn)-rich crusts that accrete in the order of few μm/ka on weathering-resistant lithologies. Although these crusts can form in all climates, they are best known in arid to semi-arid settings. Aeolian dust is understood as a major contributor to the distinct trace metal and REE enrichments in rock varnish....

We investigated rock varnish formed on sandstone and petroglyphs in the Hima area, southwestern Saudi Arabia. To characterize the rock varnish, we made in-situ measurements by portable x-ray fluorescence (pXRF) and analyzed samples by femtosecond laser-ablation inductively coupled–plasma mass spectrometry (fs LA-ICP-MS). Detailed chemical analysis...

The spatial distribution of transition metal valence states is of broad interest in the microanalysis of geological and environmental samples. An example is rock varnish, a natural manganese (Mn)-rich rock coating, whose genesis mechanism remains a subject of scientific debate. We conducted scanning transmission X-ray microscopy with near-edge X-ra...

Manganese (Mn)-rich natural rock coatings, so-called rock varnishes, are discussed controversially regarding their genesis. Biogenic and abiogenic mechanisms, as well as a combination of both, have been proposed to be responsible for the Mn oxidation and deposition process. We conducted scanning transmission X-ray microscopy - near edge X-ray absor...

Petroglyphs, engraved throughout the Holocene into rock varnish coatings on sandstone, were investigated in the Ha’il region of northwestern Saudi Arabia, at Jabal Yatib, Jubbah, and Shuwaymis. The rock art has been created by removing the black varnish coating and thereby exposing the light sandstone underneath. With time, the varnish, a natural m...

Danburite is a calcium borosilicate that forms within the transition zones of metacarbonates and pegmatites as a late magmatic accessory mineral. We present here trace element contents obtained by femtosecond laser ablation-inductively coupled plasma (ICP)-mass spectrometry for danburite from Mexico, Tanzania, and Vietnam. The Tanzanian and Vietnam...

Desert varnishes are dark rock coatings observed in arid environments and might resemble Mn-rich coatings found on Martian rocks. Their formation mechanism is not fully understood and the possible microbial involvement is under debate. In this study we applied DNA metagenomic sequencing of varnish and surrounding soil to evaluate the composition of...

Black manganese-rich crusts are found worldwide on the façades of historical buildings. In this study, they were studied exemplarily on the façade of the Freiburger Münster (Freiburg Minster), Germany, and measured in-situ by portable X-ray fluorescence (XRF). The XRF was calibrated to allow the conversion from apparent mass fractions to Mn surface...

We investigated rock varnishes collected from several locations and environments worldwide by a broad range of microanalytical techniques. These techniques were selected to address the challenges posed by the chemical and structural complexity within the micrometer- to nanometer-sized structures in these geological materials. Femtosecond laser abla...

Femtosecond laser ablation-inductively coupled plasma-mass spectrometry was used to determine the concentration of 51 elements, i.e. Li, Be, B, Na, Mg, Al, Si, K, Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Y, Zr, Nb, Ag, Cd, Sn, Sb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Pt, Au and Tl in danburites fr...

Manganese- and iron-rich materials are of major scientific and economic interest in geosciences. Many of them contain microscopic features that provide valuable information. To obtain accurate results, a homogeneous microanalytical reference material for calibration is needed. Several researchers used the Mn-and Fe-rich RMs, JMn-1, NOD-A-1, NOD-P-1...

Desert varnish describes thin, lustrous, brown to black coatings on rock surfaces of physically stable lithologies. Bulk material consists of clay minerals, cemented together by manganese and iron oxyhydroxides. Aeolian dust is considered a potential source of Mn, due to leaching processes after wetting by dew or soft rain [1]. In addition, bioenha...

Since over 200 years a number of studies have debated the origin of the black or brown, up to 250 μm thick manganese-rich coatings on rocks, referred to as rock varnish. A great amount of literature about this material has been published, but no suitable or consistent reference materials (RMs) have been established to calibrate analytical data from...

A wide range of analytical techniques were used to investigate rock varnish from different locations (Negev, Israel; Knersvlakte, South Africa; Death Valley and Mojave Desert, California): a 200 nm-femtosecond laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS), an electron probe microanalyzer (EPMA), focused ion beam (FIB) slic...

Highly resolved microanalyses are important in quite different fields of research, such as in paleoclimate research. We developed an in-situ technique for precise measurement of trace elements, trace element ratios, and isotope ratios in various environmental samples. For these investigations we used femto-and nanosecond lasers for sample ablation,...

Desert varnish is a dark and thin (< 250 µm) layer on rock surfaces. The main components are Mn and Fe oxides as well as clay minerals. We investigated the structure at the nm scale to test whether desert varnish is suitable as an archive for paleoclimate information. For these investigations we used femto-and nanosecond LA-ICP-MS and the results o...

Atmospheric dust has a substantial impact on climate, and climate in turn affects the dust flux and origin. Knowing its composition helps to determine dust sources and climate processes. We have therefore determined 46 major and trace elements in dust samples, using in-situ 200 nm-femtosecond laser ablation-inductively coupled plasma-mass spectrome...