RESPONSE  to  the  PREPRINT  of  E. Bard  and  T.J. HEATON  (B&H)

"On the tuning of plateaus in atmospheric and oceanic ¹⁴C records to derive calendar chronologies of deep-sea cores and records of ¹⁴C marine reservoir age changes"

Michael Sarnthein¹ and Pieter M. Grootes²

1 Institute of Geosciences, University of Kiel, 24118 Kiel, Germany

2 Institute of Ecosystem Research, University of Kiel, 24118 Kiel, Germany

Abstract

In response to an extended comment of Bard and Heaton (2021) (B&H) on the synthesis paper of Sarnthein et al. (2020) we counter their reservations both in the field of statistics and about the technique of ¹⁴C plateau-tuning (PT), like in a manual, one-by-one, by means of telling lines of evidence. In particular, we single out the following points of view:

-- We show proof that results of PT of marine sediment records are hardly affected by bioturbational mixing and changes in foraminifera abundance, given the limitation of PT to cores with sedimentation rates >10 cm/kyr;

-- We illustrate the importance of initial guidelines of conventional stratigraphy to confine overall sedimentation rates as boundary condition and to derive alternative modes of PT for a whole suite of ¹⁴C jumps and plateaus in a sediment record, ¹⁴C structures to be compared to those of the paired atmospheric reference record of Lake Suigetsu.

-- Extended tests (Balmer & Sarnthein, 2016) revealed that changes in sedimentation rate per se are unable to generate a complete suite of ¹⁴C plateaus by now already defined in some 20 sediment cores and independently corroborated by various lines of local evidence.

-- Over the interval 10 - 15 cal. ka, the plateau structures of the Suigetsu atmospheric (atm) ¹⁴C record are clearly paired with well-defined tree ring- and floating tree ring-based ¹⁴C structures (IntCal13; Adolphi et al., 2017). By comparison, we suggest that prior to 15 cal. ka the continuing ¹⁴C fine structure of noisy Suigetsu with ¹⁴C jumps and plateaus is by far more realistic than the admittedly smoothed ¹⁴C trend of the Hulu speleothem and IntCal20, records that may also suffer from unknown but likely changes in the Hulu Dead Carbon Fraction (DCF).

-- By comparison to Holocene and late deglacial times, where PT may be constrained by tree ring records, glacial-to-early deglacial marine reservoir ages (MRA) can indeed be regarded as largely constant over time spans as long as ¹⁴C plateaus about 500-1000 yr. In turn, major MRA changes are confined to more extended intervals of climate, sea ice cover, and ocean circulation similar to those of Heinrich events, Dansgaard Oeschger cycles, and their multiples.

-- Per analogy to the record of 10-15 cal. ka, overall ¹⁴C changes and shifts in the radiocarbon clock at 15-29 cal. ka are necessarily focused to inter-plateau times, just 18 % of the total time span as estimated by B&H. This concept indeed was first documented by means of PT.

-- We show that minor intra-plateau changes in MRA indeed exist, although they cannot be specified by our limited sampling resolution of ~50-150 yr. Careful inspection of the complete suite of plateaus in each core enabled us occasionally to identify distinct intra-plateau changes.

-- Concerns about low sampling density are unfounded. ¹⁴C structures in pelagic sediment records like boundaries of ¹⁴C plateaus, were not "under-constrained" by ¹⁴C ages but systematically documented by iterative sampling.

-- The box model discussion is scientifically correct. However, it only deals with Pla, the planktic ¹⁴C concentration of ocean surface waters, and not with MRA = (Pla-Atm).

In view of these findings the technique of PT cannot be regarded as 'result of inherent pitfalls'. Rather PT is emerging as great opportunity to generate both a suite of narrow-standing and robust age tie points for marine sediment records and a record of short-term changes in MRA and paleoceanography for last glacial-to-deglacial times in ocean sediment cores where independent high-resolution calendar age information is usually rare.

ALL DETAiLS of this response letter to cp-2020-164 are given in the attached file, moreover, in a companion letter submitted by PM Grootes and M. Sarnthein.

Comments on

Edouard Bard and Timothy J. Heaton (B&H)

On the tuning of plateaus in atmospheric and oceanic ¹⁴C records to derive calendar chronologies of deep-sea cores and records of ¹⁴C marine reservoir age changes.

Climate of the Past. Discussions

Pieter M. Grootes¹ and Michael Sarnthein²,

Institute for Eco System Research¹ and Institute of Geosciences², Christian Albrechts University Kiel, 24118 Kiel, Germany

             In the fall 2019, Edouard Bard and Tim Heaton (B&H) got access to the discussion version (opened in CPD at 25-10-2019) of our paper “Plateaus and jumps in the atmospheric radiocarbon record – Potential origin and value as global age markers for glacial-to-deglacial paleocean­ography, a synthesis” (Sarnthein et al., Clim. Past 16, 2020 (SA2020)). A letter to the Editor of CPD on 31-01-2020 stated they had written an extended comment to the paper but had submitted it as a research paper since it “includes substantial material of broad interest to the community using radiocarbon in marine sediments for geochronology and paleoceanography”. This comment is now subject of our discussion. Its aim is to demonstrate that Plateau Tuning (PT) is fraught with problems and should not be used.

            We thank B&H for the time and efforts they spent formulating the problems they see with the technique of ¹⁴C plateau tuning. Their detailed arguments and reasoning extend far down to basic processes that may control an atmospheric and sedimentary ¹⁴C record and thereby provide a base for a factual discussion of PT. Both basics and details are important when evaluating potential major-to-minor pitfalls of PT but can rarely be discussed at meetings or workshops (B&H lines 65-69). As stated in their letter the paper ‘includes substantial material of broad interest’ and many of their potential pitfalls are worth considering. Our response may help to clear various misconceptions and further explain crucial aspects of the PT method. This is important since all of us aim to find the best-possible techniques to generate proper age control of ocean sediment records and to make an optimum use of the wealth of environmental information they contain. Below we summarize two points where B&H misconstrued PT and we advocate a different conclusion from their Fig. 3 , 5, and 6.  Then we address their specific chapters and text.

Summary

Many of the 17 objections raised by B&H are based on two simple points:

• The difficulty of reliably identifying a single ¹⁴C-concentration plateau in a noisy ¹⁴C sediment record and then finding its correct partner in the noisy record of atmospheric ¹⁴C concentrations

This is the subject of eight objections (2.1; 2.2; 2.5; 2.6; 3.3; 3.4 straight and part of 2.3 and 3.1).

These objections are based on lines 40-41, the first lines of the B&H Introduction: In line 40 the term ‘a suite of’ is missing between ‘tuning’and  ‘hypothesized’ and in line 41 ‘those that’ should be replaced by ‘a suite of plateaus’.

                       The problem of how to identify a plateau has been extensively considered in the development of PT. Sarnthein et al., 2007 clearly mention they ‘identified a reference suite of prominent atmospheric ¹⁴C “plateaus” ‘, based at the time on Cariaco ODP 1002 and on U/Th dated corals and Bahama speleothem, and the identification of ‘analogous series of ¹⁴C plateaus in several other marine sediment cores …’. This emphasis on suites of plateaus and suites of tie-points has been part of every PT paper including SA 2020, discussed by B&H. As B&H point out: Identifying a single plateau is very hard. Further details on meeting this set of questions are given in the companion response text of Sarnthein & Grootes (S&G).

• The focus on ¹⁴C concentration changes in the surface ocean (pla = planktic ¹⁴C concentration) instead of on marine reservoir age (MRA = pla- Atm, where Atm is the contemporaneous atmospheric ¹⁴C concentration).

This leads to objections 2.3; 2.7; 2.8; 3.1 and 3.2 that basically repeats 2.3.

            In a simple carbon cycle box-model (e.g. Siegenthaler et al., 1980) with a deep ocean that contains about 60 times more carbon and 50 times more radiocarbon than the atmosphere, most of the variability in ¹⁴C concentration will be in the atmosphere and in its closely-connected thin ocean-atmosphere exchange layer. The focus of B&H on the surface ocean is logical, because it is easily accessible and its plankton provides our paleoenvironmental record, but MRA is the difference in ¹⁴C concentration between atmosphere and surface ocean (pla-Atm). B&H use the Bard et al., 1997, 12-box model to calculate an attenuated and somewhat delayed response of the surface ocean to, especially rapid, atmospheric ¹⁴C changes. This leads them to reject the PT derived MRA changes as "too large, too frequent, too abrupt". Their modelling addresses, however, only one facet of MRA, and a strongly attenuated pla signal will generate an MRA (= pla-Atm) signal with little attenuation. This is borne out by the effects on MRA of the ¹⁴C bomb spike and Miyake events mentioned in their text. A box model, moreover, does not consider local variations in near-surface ocean mixing and ocean-atmosphere exchange, that can lead locally to large and rapid changes in pla and thus MRA for an unchanged atmosphere.

            Fig. 3a shows the translation of the PT suite of plateaus, defined by SA2020, in the ¹⁴C-age/ calendar-age domain, into the Δ¹⁴C/calendar age domain and compares the translated plateau step curve with the Bayesian-spline generated Suigetsu atmospheric record.  The statistically sound zig-zags of the  Bayesian Suigetsu curve reveal a generally satisfactory agreement with the (green) plateaus (sections of (faster) decreasing Δ¹⁴C vs. decreasing cal. age) and jumps (sections of slower decrease or increase), despite the fact that the plateaus defined by Sarnthein et al. (2015 and 2020) were based on the 2012 Suigetsu data and did not consider the most recent age corrections of Bronk Ramsey et al., (2020). The zig-zag curve does not pin the plateau slope to zero and offers another look at the position of inflection points, so far defined by the beginning and end of zero-slope plateaus. This will be further explored for PT.

            The modelling exercise of 3.6 and 3.7 demonstrates how the statistical scatter of sampling and imperfect measurements may distort and mask underlying real signals. Yet, contrary to the stated conclusion, it offers hope in showing that at least two out of the three ‘plateau’ features of the underlying short record can indeed be found in the modelled examples. It also makes a clear point that such identification of the ‘true’ fine structure of a ¹⁴C record is a serious research project requiring consideration of a broad range of conventional age tie points and oceanographic information and a long sequence of plateaus in order to produce reliable results. And, even then, it still needs to be checked against other independent records.

My apologies for a partially failed upload. Attached now the pdf belonging to the earlier comment.

I hope we can have a good annd factual discussion.

Comment on cp-2020-164 “On the tuning of plateaus in atmospheric and oceanic ¹⁴C records to derive calendar chronologies of deep-sea cores and records of ¹⁴C marine reservoir age changes” by Edouard Bard and Timothy J. Heaton

Frank Lamy (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany)

Helge Arz (Leibniz-Institute for Baltic Sea Research, Warnemünde, Germany)

To our knowledge, Bard & Heaton provide the first thorough and independent discussion of the so-called Plateau Tuning method presented in a number of papers by Michael Sarnthein and colleagues over the past ~15 years and particularly in a recent CP “review” paper (Sarnthein et al., 2020).

As discussed by both referees (Paula Reimer and an Anonymous Referee), Bard & Heaton provide a critical assessment of the Plateau Tuning method. We mostly agree to these assessments and we therefore do not want to further discuss the principal errors and limitation as well as statistical issues of the Plateau Tuning method.

Since we both recovered and investigated sediment cores from the Chilean margin and adjacent South Pacific over the last two decades, we would like to comment and provide some more details on the controversial results on sediment records from this region as presented in Sarnthein et al. (2020) and correctly noted by Bard & Heaton. Some of these statements are repeated in CC1 by Michael Sarnthein.

Our comment primarily refers to the Plateau Tuning results of sediment core PS97/137 and some of the hypothetic implications for the sedimentary setting and paleoceanographic implications (e.g. the derived reservoir ages). Since these results are part of a manuscript in preparation, we here provide some of the background information necessary to assess the Plateau Tuning attempt and reservoir age record of PS97/137.

Site PS97/137 is located at the upper continental margin, ~30 nm off the entrance to the Strait of Magellan at a water depth of ~1100 m. PS97/137 lies within a few miles from site MD07/3128 that provided excellent high-resolution paleoceanographic records (e.g., Caniupán et al, 2011; Lamy et al. 2015.). Both sites are within a small-scale sediment depo-center (“sediment drift”) with an up to ~700 m thick sediment sequence that has been chosen for IODP drilling during Expedition 383 (Site U1542; Lamy et al., 2019). Sediments are foraminifera oozes during the Holocene underlain by glacial, primarily siliciclastic sediments with low carbonate and biogenic opal contents. These low biogenic contents are primarily due to dilution by enhanced terrigenous sediment supply from the glaciated hinterland, absence of sediment trapping in the fjords, and reduced winnowing by the overlying Cape Horn Current.

Physical properties and geochemical data allow to splice PS97/137 into the well-dated record of MD07/3128 suggesting that the here investigated part of PS97/137 covers the past ~25 ka. The latest published age model of MD07/3128 (Lamy et al., 2015) was based on 8 radiocarbon dates for the interval covered by PS97/137 using the reservoir ages derived at the Chile margin further north (MD07-3088; Siani et al., 2013). Sedimentation rates are below 10 cm/ka during the Holocene, ~15 cm/kyr during Termination 1 and in the order of ~1 m/kyr during the Last Glacial Maximum. As such, these highly variable sedimentation-rates are generally complicating the assignments of individual plateaus (assuming that these exist). This complication likely applies also to some of the earlier published Plateau Tuning results from low-latitude continental margin sites (e.g., GeoB3910, Balmer et al. 2016) with highly variable sedimentation-rates (Arz et al., 1998; Arz et al., 1999).

Ultimately, this is reflected in inconsistent Plateau Tuning results for PS97/137 in the submitted and published version of Sarnthein et al. (2020) as correctly noted by Bard and Heaton (submitted). We note that high reservoir ages (e.g., 1000 ¹⁴C yrs as based on the most recent Plateau Tuning) are not substantially higher than the 800 yrs we assumed for MD07/3128 based on the Siani et al. (2013) study. However, Sarnthein et al. (2020 and supplements to CC1) derive various hiatuses in their Plateau Tuning records, not only at our but also at other high resolution continental margin sites. Though we cannot strictly exclude hiatuses at the “sediment drift” site PS97/137, we do not expect to find significant gaps in the sedimentation during glacial periods characterized by reduced bottom currents and strong terrestrial sediment input (Lamy et al., 2015). Therefore, at least for the southern Chilean Margin, we do not find solid evidence nor see any reason to conclude: (citing CC1 by Michael Sarnthein: Though widely not appreciated by paleoceanographers, hiatuses appear to be a feature actually widespread at high-sedimentation  rate sites in the deep sea – One may assume: The higher the rates the more extreme they may be subject to changes in depositional regime”). This conclusion is at least counterintuitive and needs more in-depth investigations and supporting evidence.

Finally, regarding PS97/137, Michael Sarnthein (supplements to CC1) states “PS97-137 off Southern Chile (Küssner et al., 2020): A rough count of sediment laminations has fairly well confirmed the length of a PT-derived paired 14C plateau for the LGM”. Since the lamination texture is still under debate and in neighboring cores (MD07/3128, IODP U1542) this feature is not present, more thorough investigations than “rough counting” is required.

As mentioned above, we are strongly in favor of assessing the Plateau Tuning method independently and generally concur with this critical discussion of the Plateau Tuning method by Bard & Heaton, who provide an important separate assessment for those who seek in-depth information on the applicability, reliability and limitations of the Plateau Tuning method. Further independent studies on well suited high resolution, continuous sediment records are in our opinion still required to put the Plateau Tuning method on the touchstone.

References:

Arz, H. W., J. Pätzold, and G. Wefer, 1998. Correlated millennial-scale changes in surface hydrography and terrigenous sediment yield inferred from last-glacial marine deposits off northeastern Brazil. Quaternary Research 50, 157-166.

Arz, H. W., Pätzold, J. & Wefer, G., 1999. Climatic changes during the last deglaciation recorded in sediment cores from the Northeast Brazilian Continental Margin. Geo Marine Letters, 19: 209-218.

Balmer, S., Sarnthein, M., Mudelsee, M., Grootes, P.M., 2016. Refined modeling and C-14 plateau tuning reveal consistent patterns of glacial and deglacial C-14 reservoir ages of surface waters in low-latitude Atlantic. Paleoceanography 31, 1030-1040.

Caniupán, M., Lamy, F., Lange, C.B., Kaiser, J., Arz, H., Kilian, R., Urrea, O.B., Aracena, C., Hebbeln, D., Kissel, C., Laj, C., Mollenhauer, G., Tiedemann, R., 2011. Millennial-scale sea surface temperature and Patagonian Ice Sheet changes off southernmost Chile (53 degrees S) over the past similar to 60 kyr. Paleoceanography 26, PA3221.

Lamy, F., Arz, H.W., Kilian, R., Lange, C.B., Lembke-Jene, L., Wengler, M., Kaiser, J., Baeza-Urrea, O., Hall, I.R., Harada, N., Tiedemann, R., 2015. Glacial reduction and millennial-scale variations in Drake Passage throughflow. P Natl Acad Sci USA 112, 13496-13501.

Lamy, F., 2016. The expedition PS97 of the research vessel POLARSTERN to the Drake Passage in 2016. Reports on polar and marine research. Bremerhaven, Germany. https://doi.org/10.2312/BzPM_0702_2016

Lamy, F., Winckler, G., Alvarez Zarikian, C.A., and the Expedition 383 Scientists, 2019. Expedition 383 Preliminary Report: Dynamics of the Pacific Antarctic Circumpolar Current.International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.383.2019.

Sarnthein, M., Kussner, K., Grootes, P.M., Ausin, B., Eglinton, T., Muglia, J., Muscheler, R., Schlolaut, G., 2020. Plateaus and jumps in the atmospheric radiocarbon record - potential origin and value as global age markers for glacial-to-deglacial paleoceanography, a synthesis. Clim Past 16, 2547-2571.

Siani, G., Michel, E., De Pol-Holz, R., DeVries, T., Lamy, F., Carel, M., Isguder, G., Dewilde, F., Lourantou, A., 2013. Carbon isotope records reveal precise timing of enhanced Southern Ocean upwelling during the last deglaciation. Nature Communications 4.

Comment on cp-2020-164 “On the tuning of plateaus in atmospheric and oceanic ¹⁴C records to derive calendar chronologies of deep-sea cores and records of ¹⁴C marine reservoir age changes” by Edouard Bard and Timothy J. Heaton

Elisabeth Michel (LSCE, IPSL, CNRS-CEA-UVSQ, FRANCE)

Giuseppe Siani (GEOPS, Université Paris-Saclay, France)

Precise dating of marine cores is a tricky problem even during the period covered by ¹⁴C dating because of varying ¹⁴C reservoir age (MRA) of the surface ocean (i.e. Bard et al., 1988, Bard et al., 94, Siani et al., 2001; Bondevick et al., 2006; Austin et al., 2011 ; Skinner et al., 2014). It is now well admitted that even in subtropical surface waters the MRA will change, at least because of varying atmospheric CO₂ (Galbraith et al., 2015). Thus any new approach that would help to establish precise marine core chronology would be most welcome. Hence, transposing the “wiggle–matching technique” to marine cores could be a nice idea. However Bard and Heaton indicate in this paper that, until now, atmospheric ¹⁴C wiggles are precisely determined only before 12.6 kyrs (Intcal 20, Reimer et al., 2020). Furthermore, as Bard and Heaton highlight, marine sediment cores are generally not varved and many mechanisms, such as varying MRA, bioturbation, might create wiggles in their ¹⁴C record that do not have any atmospheric ¹⁴C plateau counterpart. Thus “wiggle-matching” for marine sediment cores could only be a further constrain to other chronology/sedimentation rate determinations for high sedimentation rate cores with a ~100 years resolution ¹⁴C record.  

Unfortunately the only rigorous determination of past surface ocean MRA requires to measure the ¹⁴C difference between atmospheric and a marine record. This can be achieved for well identified common horizons like a volcanic ash layer on land and in marine sediments. This approach is certainly not trivial, it requires the comparison of the tephra chemistry (including very often major and trace elements geochemical signatures) and requires other constrains based on a thorough stratigraphy framework of both marine and continental archives.

Sarnthein et al., (2020) present the results of MRA with the marine plateau technique for core MD07-3088 in the South-East Pacific, a core that we studied previously. This core presents high sedimentation rate, ~60cm/kyrs during the Holocene and deglaciation and ~300cm/kyrs during the last glacial maximum, depending from a peculiar sedimentary dynamics via the terrigenous contribution from the nearby fjords and from the closeness of the Patagonian ice sheet to the study site during the last glacial period (Davies et al., 2020). Few tephra horizons were determined within this core (Siani et al., 2013; Haddam et al., 2018), and chemically and stratigraphically connected to land dated eruption of the Hudson Volcano. Thus for the deglaciation part, this core benefit from a determination of four independent surface water MRA with a 100-150 yrs error on the age determination. Sarnthein et al., (2020) present in the supplementary material a curve of the MRA for this core from 22 to 10.5 kyrs. The plateau tuning does not require any hiatuses in this core but sediment rate changes up to a factor of 25 that is the result of considering large plateau and not comparing the full wiggles observed in the intcal20 calibration curve as highlighted by Bard and Heaton (wiggles from 10 to 17 cal kyrs “plateaus pre boreal to plateau 2b”). Sarnthein et al., (2020) recognize that the MRA could change within a plateau as they associated two different MRA for this core within the plateau 1 period. For the deglaciation part of this high sedimentation rate core with a high resolution ¹⁴C curve, it could be interesting to realize a real wiggle-matching exercise using an ocean circulation model, to take into account the different shape of the marine and atmospheric curves and test different MRA changes, to check if this technique could add any valuable constrain on the MRA changes. For the glacial part of the core (~17-22 kyrs), that has no other constrains from tephra layers, and for which the benthic and planktonic isotopic curve do not show any variations within stage 2, the choice of the possible wiggles is widely open, if the wiggles were clearly determined in intcal20 calibration curve. Thus the MRA cannot be precisely defined, the errors could be two to three times the MRA changes for this core.

Up to now there are only few determination of MRA, benefiting either of ¹⁴C dated marine shell with known collection age date, volcanic eruption for which tephra are dated on land and in marine cores. A common practice is to use tuning of climatic records between continental and marine archives, like rapid changes in temperature (Austin et al., 2011). It is important to be aware of the assumptions under such exercise and to assign corresponding errors on the chronology obtained for marine records. Such technique however will mask possible nonsynchronous climate changes and precludes studying the climate dynamics on the centennial to pluri-decennal time scale (Mekhaldi et al., 2020).

Whatever the method proposed to assign reliable and precise chronology for marine records, it is needed to assess its applications and limitations with a rigorous theoritical and experimental study. Bard and Heaton provides a careful study of the different assumptions underlying the “plateau technique” that is of great importance for paleoceanographic community.

Austin, William E.N. Richard J. Telford, Ulysses S. Ninnemann, Louise Brown, Lindsay J. Wilson,David P. Small, Charlotte L. Bryant 2011. North Atlantic reservoir ages linked to high Younger Dryas atmospheric radiocarbon concentrations. Global and Planetary Change 79 226–233.

Bard, E. Correction of accelerator mass spectrometry 14C ages measured on planktonic foraminifera: Paleoceanographic implications. Paleoceanography, 3, 635-645. doi:10.1029/PA003i006p00635, 1988.

Bard, E., Arnold, M., Mangerud, J., Paterne, M., Labeyrie, L., Duprat, J., Mélières, M., Sonstegaard, E., Duplessy, J.C. 1994 - The North Atlantic atmosphere-sea surface ¹⁴C gradient during the Younger Dryas climatic event. Earth and Planetary Science Letters 126, 275-287.

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Sarnthein, M., Kussner, K., Grootes, P.M., Ausin, B., Eglinton, T., Muglia, J., Muscheler, R., Schlolaut, G., 2020. Plateaus and jumps in the atmospheric radiocarbon record - potential origin and value as global age markers for glacial-to-deglacial paleoceanography, a synthesis. Clim Past 16, 2547-2571.

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Siani, G., Michel, E., De Pol-Holz, R., DeVries, T., Lamy, F., Carel, M., Isguder, G., Dewilde, F., Lourantou, A., 2013. Carbon isotope records reveal precise timing of enhanced Southern Ocean upwelling during the last deglaciation. Nature Communications 4.