Radiocarbon Age Offsets Between Two Surface Dwelling Planktonic Foraminifera Species During Abrupt Climate Events in the SW Iberian Margin

Abstract

This study identifies temporal biases in the radiocarbon ages of the planktonic foraminifera species Globigerina bulloides and Globigerinoides ruber (white) in a sediment core from the SW Iberian margin (so‐called Shackleton site ). Leaching of the outer shell and measurement of the radiocarbon content of both the leachate and leached sample enabled us to identify surface contamination of the tests and its impact on their 14 C ages. Incorporation of younger radiocarbon on the outer shell affected both species and had a larger impact downcore. Interspecies comparison of the 14 C ages of the leached samples reveal systematic offsets with 14 C ages for G. ruber being younger than G. bulloides ages during the last deglaciation and part of the Early and mid‐Holocene. The greatest offsets (up to 1,030 years) were found during Heinrich Stadial 1, the Younger Dryas, and part of the Holocene. The potential factors differentially affecting these two planktonic species were assessed by complementary 14 C, oxygen and carbon isotopes, and species abundance determinations. The coupled effect of bioturbation with changes in the abundance of G. ruber is invoked to account for the large age offsets. Our results highlight that 14 C ages of planktonic foraminifera might be largely compromised even in settings characterized by high sediment accumulation rates. Thus, a careful assessment of potential temporal biases must be performed prior to using 14 C ages for paleoclimate investigations or radiocarbon calibrations (e.g., marine calibration curve Marine13, Reimer et al., 2013, https://doi.org/10.2458/azu_js_rc.55.16947 ).

Marine Reservoir Age Variability Over the Last Deglaciation: Implications for Marine CarbonCycling and Prospects for Regional Radiocarbon Calibrations

“…Overall, these results underline the potential importance of large changes in the ocean's "disequilibrium DIC" pool despite its relatively small total inventory in the modern ocean, with implications for the mechanisms responsible for past CO 2 change on both millennial-and glacial-interglacial time scales. (Skinner et al, 2003(Skinner et al, , 2014 and SHAK-6-5 K (Ausín et al, 2019). The marine records are compared to the event stratigraphy expressed in the NGRIP dust concentration record (Ruth et al, 2003).…”

“…By way of illustration, we demonstrate the application of a tentative regional radiocarbon calibration for the Iberian Margin based on a cubic spline fit to our collected R‐ages from this region, which we have added to the Intcal13 atmospheric radiocarbon curve (Reimer et al, ) to derive a putative Iberian Margin surface ocean curve. Applying this regional calibration curve to a particularly well‐dated sediment core that was not included in the compiled data set (SHAK‐6‐5K; Ausín et al, ) yields an age model that is arguably more accurate than that obtained by assuming constant R‐ages across the deglaciation. Accordingly, Figure shows how stadial/interstadial transitions recorded in Globigerina bulloides δ 18 O closely track their counterparts in the independently dated NGRIP Greenland ice‐core dust concentration record.…”

“…Comparison of chronostratigraphies obtained for Globigerina bulloides δ 18 O measured in two sediment cores from the Iberian Margin: MD99‐2334 K (Skinner et al, , ) and SHAK‐6‐5 K (Ausín et al, ). The marine records are compared to the event stratigraphy expressed in the NGRIP dust concentration record (Ruth et al, ).…”

Climatic subdivision of Heinrich Stadial 1 based on centennial-scale paleoenvironmental changes observed in the western Mediterranean area

“…The presented age offsets between records could be related with variations in reservoir ages of the Atlantic and Mediterranean promoted by thermohaline circulation collapse in both areas during HS1 (Sierro et al, 2005). In addition, the significant decrease in the atmospheric 14 C between 17.5 and 14.5 kyr also difficult age models during this period (Broecker and Barker, 2007), whereas dating on different foraminifera species can also produce large differences on radiocarbon ages, especially during HS1 (up to 1000 years) (Ausín et al, 2019). The asynchronicity and the early record of HS1 in Padul (18.4-15.6 kyr BP) with respect to the equivalent GS-2.1a cold event from Greenland ice-core records (17.5-14.7 kyr BP) is evident.…”

Millennial‐Scale Age Offsets Within Fossil Assemblages: Result of Bioturbation Below the Taphonomic Active Zone and Out‐of‐Phase Production

“…Several mechanisms can explain between‐species age offsets (Barker et al, ; Mekik, ; Paull et al, ), including (1) temporal differences in species production occurring under very low sedimentation and/or high sediment mixing by bioturbation, leading to age offsets that exceed the duration over which species change in abundance (Ausín et al, ; Bard, ; Broecker et al, ; Loubere, ; Löwemark & Grootes, ; Manighetti et al, ; Peng & Broecker, ), and/or (2) differences in disintegration between fragile and robust species, paralleling age mismatches between 210 Pb and 14 C tracers (Barker et al, ; Broecker et al, ; Broecker & Clark, ; DuBois & Prell, ; Peng et al, ). Mixing and disintegration rates, both integral in the generation of age offsets, have potential to be coupled: bioturbational mixing of sediment and bioirrigation enhance aerobic degradation of organic matter and shift the redox boundary downward (Kristensen, ), affecting the preservation of carbonate shells that respond to changes in pore‐water chemistry (Aller, ).…”