The ELSA-20 record is a time series of organic carbon content in four sediment cores, which were analyzed with 1 mm resolution directly on the surface of the drill cores using the ISRS670 method (In Situ Reflectance Spectroscopy). Sedimentation rate of the cores is 1-5 mm/year; accordingly, the raw ISRS670 data have a resolution of months up to a year, continuously from today all the time back to 60,000 yr b2k. The last millennium was analyzed on SMfreeze, the Holocene on cores from Holzmaar (HM3, HM4), the glacial time from 14,700 – 60,000 yr b2k was analyzed on two cores from Auel (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 by Rein & Sirocko 2002. The absorption at 670 nm was calibrated in the first study versus organic carbon and chlorin content, both of which revealed a significant relation to the absorption at 670 nm, which is subtracted from the interpolated value between 640 and 730 nm. The ISRS670 detects accordingly chlorophyll ab, 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 the Corg content. The relation is apparently linear and can be expressed by the simple equation: Corg [%] = ISRS670 x 22.7. See Sirocko et al. (2021) for more information on the method. The ISRS670 detects only the organic carbon derived from diatoms and other photosynthetic active algae. Biomass from leaves, however, are not detected with the method, because chlorophyll and its derivatives deteriorate completely during aging. This deterioration is shown by the spectral signal of samples from Carpinus betulus and Pinus sylvestris, which reveal an ISRS670 signal from the green leaves and needles, but no absorption at 670 nm from the withered leaves and needles.