The annual resolution ISRS670 method to quantify organic carbon content

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The records of ELSA-20 were analyzed with 1 mm resolution directly on the surface of the drill cores using the ISRS670 method, which was originally developed by Rein & Sirocko (2002). The sedimentation rate of the sediment cores is 1-3 mm/year; accordingly, the raw ISRS670 data have a resolution of several months up to a year from today all the time back to 60,000 years before the year 2000. The data for the time 14,700 – 60,000 BP were analysed on the two cores AU3 and AU4.

ISRS670 was determined with a Gretag Spectrolino, which measures the reflectance for each wavelength of the visible light as percent relative to a white color standard. The Spectrolino measures the sediment reflectivity over an area 2.5-mm wide; the individual measurements were done in 1 -mm increments. The Spectrolino technique was originally developed for marine sediments (Rein & Sirocko, 2002). The absorption at 670 nm was calibrated in the first study versus organic carbon and chlorine content, both of which revealed a significant relation to the “In situ Reflectance Spectroscopy – ISRS” absorption at 670 nm, which is subtracted from the interpolated value between 640 and 730 nm. The ISRS670 detects accordingly chlorophyll a, b, and c and also bacteriochlorophyll c and d (Green sulfur bacteria) as well as their derivates and can be applied to detect trends in total aquatic paleoproduction in both ocean and lake sediments.

Absorption forms a pronounced minimum in the wavelength range of 640 nm – 730 nm in cores both from Holzmaar and Auel. Reflectivity in this wavelength band (I-Band, 660 – 670 nm, i.e. the red part of the spectrum), was shown to relate to sediment concentrations of chlorophyll a and its degradation products.

The ISRS670 of AU3 and AU4 was calibrated to Corg (TOC) (Sirocko et al., 2021). The relation is apparently linear and can be expressed by the simple equation: Corg [%] = ISRS670 x 22.7. This equation was used to convert the 200,000 ISRS670 measurements (100,000 each in core AU3 and AU4) into a final Corg concentration time series. The results implicate that the vast majority of organic matter in the Holzmaar lake is from bioproduction in the lake itself.

In leaves, however, chlorophyll and its derivatives generally decrease during aging. The decrease, including their complete loss, was also shown by the spectral signal of the samples of Carpinus betulus and Pinus sylvestris, which reveal an ISRS670 signal from the green leaves and needles, but no adsorption at 670 nm from the withered leaves and needles. Accordingly, the amounts of biomass from terrestrial plants are not detected with the ISRS670 method. The ISRS670 signal is thus solely derived from remnants of chlorophyll, i.e. chlorins, and we thus call our proxy Corg(chlorins) to differentiate from the bulk Corg fraction.