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

Abstract

Abstract. Oxygen isotope (δ18O) 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 δ18O to temperature and water δ18O, a reasonably accurate estimation of water δ18O, and due consideration of growth-rate effects. Taking these factors into account, δ18O 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 bivalve 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 ∘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 ∘C nearby). Using a model-derived estimate of water δ18O (0.0 ‰), summer surface temperature was initially in the cool temperate range (<20 ∘C) and then (during the Mid-Piacenzian Warm Period; MPWP) increased into the warm temperate range (>20 ∘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 ∘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 ∘C higher than the present value (10.9 ∘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 (Δ47) and biomineral unit thermometry offer means of checking the δ18O-based temperatures.

The fossil bivalve Angulus benedeni benedeni: a potential seasonally resolved stable isotope-based climate archive to investigate Pliocene temperatures in the southern North Sea basin

“…Some studies indicate generally cool winter conditions, with only slightly higher summer temperatures and a slightly larger seasonal range compared to today (Vignols et al 2019). Other studies suggest cool winter conditions similar to today with significant warming during summer, yielding significantly larger seasonal temperature ranges of up to 17°C (Raffi et al 1985;Johnson et al 2009;Valentine et al 2011;Johnson et al 2022). Yet another range of studies suggest strong warming in both summer and winter (Strauch 1968;Head 1997;1998).…”

“…Secondly, short growth breaks during winter, as suggested by the dark bands present on the exterior shell surface, may have prevented the coldest temperatures from being recorded in A. benedeni benedeni. Thirdly, summer stratification during deposition of the lower Oorderen Member at 40-50 m water depth was found to be a likely possibility by previous research on bivalves from this member (Valentine et al 2011;Johnson et al 2022). Previous research on the somewhat older Coralline Crag Formation also found evidence for summer stratification in the early Pliocene (Jenkins and Houghton 1987;Johnson et al 2009).…”

“…Previous studies have indicated good preservation for bivalves from the Oorderen Member (Valentine et al 2011;Johnson et al 2022), from other members of the Lillo Formation (Johnson et al 2022) and from the early Pliocene Ramsholt Member of the Coralline Crag Formation (Johnson et al 2009;Vignols et al 2019). Pliocene shells from the North Sea basin are expected to yield decent temperature results by geochemical analysis, as long as preservation is not obviously suspected (e.g., in the Sudbourne Member of the Coralline Crag Formation in southeast England, whose aragonitic shells have nearly all been dissolved; Balson et al 1993).…”

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

“…in the alphabetical order of parts), the sequence in each figure is as in Table 1, read top to bottom. The raw data are available online (Johnson et al, 2021c).…”

“…Data availability. The raw isotope and increment data, and the corresponding shell heights, are available in the open-source online repository Zenodo (https://doi.org/10.5281/zenodo.5585630; Johnson et al, 2021c).…”