Defining the timing and duration of the Kačák Interval within the Eifelian/Givetian boundary GSSP, Mech Irdane, Morocco, using geochemical and magnetic susceptibility patterns

Here we present new geochemical and magnetic susceptibility (MS) data from the Global boundary Stratotype Section and Point (GSSP) for the Middle Devonian Eifelian–Givetian (E–G) boundary at Mech Irdane in the Anti-Atlas Mountains of eastern Morocco. These data come from 395 samples collected over a stratigraphic interval of ~ 20 m, beginning ~ 4.5 m below the boundary and extending to the Upper pumilio marker bed ~ 14.5 m above the boundary. MS data show long-term transgressive–regressive (T/R) eustatic cyclicity upon which is superimposed shorter-term climate cyclicity. Fourier analysis of these data yield high power at four frequencies that are consistent with Milankovitch–band orbital forcing: eccentricity (E1 and E2) at ~ 400 and ~ 100 kyr, obliquity (O1) at ~ 32 kyr, and precession (P2) at ~ 20 kyr, the latter two corrected for secular changes in Earthʹs orbit since the Middle Devonian. A floating-point time scale (FPTS) for the boundary interval has been developed using a P2 cyclicity that allows temporal resolution to ~ 10 kyr within the section. Based on the stratigraphic ranges of geochemical and MS anomalies, as well as previously reported biostratigraphic extinctions from the boundary interval, we estimate the Kačák Interval to extend from ~ 1.0 m below the E–G boundary to ~ 1.0 m above it, with a duration of.~200 ± 10 kyr. Geochemical proxies demonstrate that the Kačák Interval was characterized by more reducing conditions, greater organic carbon burial, and an increase in the flux of detrital materials (especially clays), reflecting an increase in subaerial weathering rates. The pumilio beds, located ~ 12–15 m above the E–G boundary, were deposited during successive maximum highstands under conditions of elevated primary productivity and reduced detrital influx.