Sclerochronological evidence of pronounced seasonality from the late Pliocene of the southern North Sea basin and its implications

Oxygen isotope (delta O-1(8)) sclerochronology of benthic marine molluscs provides a means of reconstructing the seasonal range in seafloor temperature, subject to use of an appropriate equation relating shell delta O-1(8) to temperature and water delta O-1(8), a reasonably accurate estimation of water delta O-1(8), and due consideration of growth-rate effects. Taking these factors into account, delta O-1(8) data from late Pliocene bivalves of the southern North Sea basin (Belgium and the Netherlands) indicate a seasonal seafloor range a little smaller than now in the area. Microgrowth-increment data from Aequipecten opercularis, together with the species composition of the bi-valve assemblage and aspects of preservation, suggest a setting below the summer thermocline for all but the latest material investigated. This implies a higher summer temperature at the surface than on the seafloor and consequently a greater seasonal range. A reasonable (3 degrees C) estimate of the difference between maximum seafloor and surface temperature under circumstances of summer stratification points to seasonal surface ranges in excess of the present value (12.4 degrees C nearby). Using a model-derived estimate of water delta O-1(8) (0.0 %0), summer surface temperature was initially in the cool temperate range (< 20 degrees C) and then (during the Mid-Piacenzian Warm Period; MPWP) increased into the warm temperate range (> 20 degrees C) before reverting to cool temperate values (in conjunction with shallowing and a loss of summer stratification). This pattern is in agreement with biotic-assemblage evidence. Winter temperature was firmly in the cool temperate range (< 10 degrees C) throughout, contrary to previous interpretations. Averaging of summer and winter surface temperatures for the MPWP provides a figure for annual sea surface temperature that is 2-3 degrees C higher than the present value (10.9 degrees C nearby) and in close agreement with a figure obtained by averaging alkenone and TEX86 temperatures for the MPWP from the Netherlands. These proxies, however, respectively, underestimate summer temperature and overestimate winter temperature, giving an incomplete picture of seasonality. A higher annual temperature than now is consistent with the notion of global warmth in the MPWP, but a low winter temperature in the southern North Sea basin suggests regional reduction in oceanic heat supply, contrasting with other interpretations of North Atlantic oceanography during the interval. Carbonate clumped isotope (Delta(47)) and biomineral unit thermometry offer means of checking the delta O-1(8)-based temperatures.

Automated summary

Oxygen isotope sclerochronology is a useful method for reconstructing seasonal seafloor temperature range. By analyzing δO-1(8) data from bivalves in the southern North Sea basin, it was found that the current range is slightly smaller compared to the late Pliocene. Surface temperature was higher than seafloor temperature during summer, resulting in a greater seasonal range.