Shell chemistry of the boreal Campanian bivalve Rastellum diluvianum (Linnaeus, 1767) reveals temperature seasonality, growth rates and life cycle of an extinct Cretaceous oyster

Abstract. The Campanian age (Late Cretaceous) is characterized by a warm greenhouse
climate with limited land-ice volume. This makes this period an ideal target
for studying climate dynamics during greenhouse periods, which are essential
for predictions of future climate change due to anthropogenic greenhouse gas
emissions. Well-preserved fossil shells from the Campanian (±78 Ma)
high mid-latitude (50∘ N) coastal faunas of the Kristianstad Basin
(southern Sweden) offer a unique snapshot of short-term climate and
environmental variability, which complements existing long-term climate
reconstructions. In this study, we apply a combination of high-resolution
spatially resolved trace element analyses (micro-X-ray
fluorescence – µXRF – and laser ablation inductively coupled plasma mass
spectrometry – LA-ICP-MS),
stable isotope analyses (IRMS) and growth modeling to study short-term
(seasonal) variations recorded in the oyster species Rastellum diluvianum from the Ivö Klack
locality. Geochemical records through 12 specimens shed light on the
influence of specimen-specific and ontogenetic effects on the expression of
seasonal variations in shell chemistry and allow disentangling vital effects
from environmental influences in an effort to refine paleoseasonality
reconstructions of Late Cretaceous greenhouse climates. Growth models based
on stable oxygen isotope records yield information on the mode of life,
circadian rhythm and reproductive cycle of these extinct oysters. This
multi-proxy study reveals that mean annual temperatures in the Campanian
higher mid-latitudes were 17 to 19 ∘C, with winter minima of
∼13 ∘C and summer maxima of 26 ∘C,
assuming a Late Cretaceous seawater oxygen isotope composition of
−1 ‰ VSMOW (Vienna standard mean ocean water). These results yield smaller latitudinal
differences in temperature seasonality in the Campanian compared to today.
Latitudinal temperature gradients were similar to the present, contrasting
with previous notions of “equable climate” during the Late Cretaceous. Our
results also demonstrate that species-specific differences and uncertainties
in the composition of Late Cretaceous seawater prevent trace element proxies
(Mg∕Ca, Sr∕Ca, Mg∕Li and Sr∕Li) from being used as reliable temperature proxies
for fossil oyster shells. However, trace element profiles can serve as a
quick tool for diagenesis screening and investigating seasonal growth
patterns in ancient shells.



23
The Campanian age (Late Cretaceous) is characterized by a warm greenhouse climate with limited land 24 ice volume. This makes this period an ideal target for studying climate dynamics during greenhouse periods, 25 which are essential for predictions of future climate change due to anthropogenic greenhouse gas    The incorporation of trace elements and carbon isotopes into bivalve shells is influenced by so-called vital 120 effects: biological controls on the incorporation of elements in the shell independent of the environment 121 (Weiner and Dove, 2003;Gillikin et al., 2005). These vital effects have been shown to mask the 122 characteristic relationships between shell trace element chemistry and the environment and appear to be 123 distinct not only between different bivalve species but also between specimens of different ontogenetic age 124 (Freitas et al., 2008). Differences between bivalve families mean that the trace element chemistry of some The key to disentangling vital effects from recorded environmental changes lies in the 132 application of multiple proxies and techniques on the same bivalve shells (the "multi-proxy approach"; e.g. 133 Ullmann et al., 2013;de Winter et al., 2017a;2018) and to base reconstructions on more than one shell 134 (Ivany, 2012 Fig. 4, Fig. 6, Fig. 8 and             were better than 0.99 and the standard deviation of reproducibility for elemental concentrations was better 273 than 5% relative to the mean value. Individual LA-ICP-MS measurements were inspected for diagenetic 274 alteration or contamination by detrital material using the same thresholds as used for XRF data (see above).

275
LA-ICP-MS and µXRF measurements were combined to cover a wider range of elements, since some 276 elements (e.g. S and Sr) were measured more reliably using µXRF, while others (e.g. Li or Ba) could only

288
Instruments, UK). The sample size (50-100 µg) allowed duplicate measurements to be carried out regularly 289 (roughly once every 30 samples) to assess reproducibility. Samples were digested in 104% phosphoric 290 acid at a constant temperature of 70°C and the resulting CO2 gas was cryogenically purified before being 291 led into the IRMS through a dual inlet system. Isotope ratios were corrected for instrumental drift and         Results of strontium isotope analyses are given in S8. The mean strontium isotope ratio of all R. diluvianum 370 specimens is 0.707552 (±0.000112; 95% confidence level). The compilation of 87 Sr/ 86 Sr results from 12 371 specimens of R. diluvianum (Fig. 4)    Si and lower Sr concentrations ( Fig. 3f-h). Foliated calcite layers are more densely packed on the inside of 385 the shell, especially in the region of the adductor muscle scar, and at the shell hinge (Fig. 3a). They are 386 characterized by high Sr concentrations and low concentrations of Mn, Fe and Si (Fig. 3f-h; S2). Further 387 away from the shell hinge and the inside of the valve, porous carbonate layers become more dominant. In 388 these regions, µXRF mapping also clearly shows that detrital material (high in Si and Fe) is often found 389 between the shell layers (Fig. 3f). SEM images show that the shell structure of R. diluvianum strongly 390 resembles that of modern oyster species, as described in previous studies (Fig. 3b-    below -3‰ and up to 0‰ VPDB (Fig. 6a). Some shells, such as R. diluvianum 3 (Fig. 7), exhibit longer 451 term trends on which these periodic variations are superimposed. These trends suggest the presence of 452 multi-annual cyclicity with a period in the order of 10-20 years, but the length of R. diluvianum records (max.   (Fig. 6b). In many parts of R. diluvianum shells, there is a clear 461 covariation between the two isotope ratios, suggesting  13 C is affected by seasonal changes. However, in

467
Due to the clear seasonal patterns in  18 O records (Fig. 6a, Fig. 7), modelled  18 O profiles closely 468 approximated the measured  18 O profiles (total R 2 = 0.86, N = 412, see S5 and S9), lending high confidence 469 to shell age models (see example in Fig. 7). Modelling allowed all proxies measured in the shells of R. 470 diluvianum to be plotted against shell age, clearly revealing the influence of seasonal variations in 471 environmental parameters on shell chemistry (S10). The age models reveal clear, statistically significant (p 472 < 0.05) ontogenetic trends in Mg/Li, Sr/Li and  13 C in nearly all specimens of R. diluvianum (see Table 1).

473
In 3 out of 5 shells,  13 C increases with age (see Fig. 6b and Table 1) with ontogenetically older specimens 474 (e.g. R. diluvianum #2) yielding overall higher  13 C values (Fig. 6b). The distribution of slopes of ontogenetic 475 trends in Li/Ca cannot be distinguished from random variation. Therefore, no widely-shared ontogenetic 476 trends were found for Li-proxies in R. diluvianum shells.

524
Unfortunately, these rarely cover the time interval in which the Ivö Klack sediments were deposited (e.g.

525
Wendler, 2013; Perdiou et al., 2016). Strontium isotope dating places the Ivö Klack deposits at 78.14 ± 0.26 526 Ma (Fig. 4), slightly above the early/late Campanian boundary, while the B. mammilatus biozone is defined 527 as late early Campanian (Wendler, 2013). Influx of radiogenic strontium-rich weathering products from the 528 nearby Transscandinavian Igneous Belt may bias age estimates from strontium isotope ratios (Högdahl et    540 diluvianum shells (Fig. 8) show that it is not straightforward to interpret these records in terms of 541 temperature changes. Some of this difficulty arises from the large inter-shell variability in trace element proxies are likely strongly affected by the specimen-specific ontogenetic trends in Li/Ca described in Table   554 1. This, together with the large inter-specimen variability shows that both Li-proxies cannot be used as 555 temperature proxies in R. diluvianum. An annual stack of all proxies shows that the positive correlation 556 between Mg/Ca and  18 O (Fig. 9) is opposite to the temperature-relationships found in modern oyster  (Fig. 8-9; see also S6). This allows Sr/Ca ratios to be used together with microstructural  Figure 9: Composite of multi-proxy records from all R. diluvianum shells stacked and plotted on a common time axis of 1 year to illustrate the general phase relationships between various proxies in the shells. Records were colored as in Fig. 7. Annual stacks plotted in this figure were produced by applying age models on all multi-proxy records, plotting all results against their position relative to the annual cycle and applying 20 point moving averages.

602
An annual stack of all R. diluvianum proxy records shows a  18 Oc-based temperature variability in Ivö Klack 603 of 16-21°C when assuming a constant  18 Osw of -1‰VSMOW (Fig. 6). However, comparison with  18 O-604 seasonality in individual specimens shows that the annual stack severely dampens seasonality due to small 605 phase shifts in maximum and minimum temperatures, small uncertainties in the age models between years 606 and specimens, and inter-annual differences and longer-term trends in temperature (see Fig. 6). A more 607 accurate estimate of the seasonal extent is obtained by calculating the seasonal range from the coolest 608 winter temperatures (12.6°C in R. diluvianum 4; Fig. 6; S10) with the warmest recorded summer 609 temperature (26°C in R. diluvianum 1; Fig. 6; S10 which yields an extreme maximum seasonal sea surface

645
Modelled growth rates in R. diluvianum peak near the end of the low temperature season and average 646 growth rates are lowest shortly after the temperature maximum (Fig. 9). This phase shift between 647 temperature and growth rate could indicate that growth in R. diluvianum in this setting was not limited by 648 low temperatures, as observed in modern mid-to high-latitude oysters (Lartaud et al., 2010). High 649 temperature extremes (>25°C) may have slowed or stopped growth, as recorded in modern low latitude 650 settings (Surge et al., 2001). Heat shock has been shown to limit the growth of modern oysters (Crassostrea 651 gigas; Li et al., 2007), although the relatively moderate SST seasonality suggests that very high (>25°C) 652 temperatures were not common at the Ivö Klack locality (Fig. 6). In this respect it is important to recognise   Growth curves of individual R. diluvianum specimens clearly converge to a general growth development 674 curve for the species (Fig. 10)

704
The similarity between growth curves of different specimens allows a von Bertalanffy curve to be fitted to the data with high confidence.

705
Sinusoidal patterns superimposed on all growth curves are caused by seasonal variability in growth rate (see Fig. 6-7). Data

709
To study variability in minimum growth temperature (Fig. 11a), length of the growth season and time of

710
year at which maximum growth occurs (Fig. 11b), we isolated individual growth years from all age models 711 of the five shells in which δ 18 O curves were measured (Fig. 11). shell at its precise position relative to the seasonal temperature cycle showed in which season the individual 714 settled and started growing its shell (Fig. 11b). All data used to create plots in Fig. 11 are provided in S14.

715
Relationships between these growth parameters are summarized in Table 2

728
The growing season is shorter than 365 days in all but five modelled years, demonstrating that growth stops 729 or slowdowns did occur in R. diluvianum. Minimum growth temperatures (temperatures at which growth 730 stopped) are concentrated around 17°C (χ 2 = 0.0088; Fig. 11a) and correlate strongly to MAT (R 2 =0.57;  Fig. 9).

739
The season of first growth (after larval stage) is concentrated in the two last months before the  18 O 740 maximum (first half of winter) when modelled water temperatures are ±17°C (Fig. 11b). Note that only three 741 of the five shells allowed sampling of the first month of growth, and extrapolated records for the other two 742 shells are likely unreliable. Comparing Fig. 11a and Fig. 11b shows that growth halts and settling occur at

776
Shell growth in R. diluvianum may not have been governed by temperature, but rather by changes in 777 productivity. The observation that peak growth rates and settling both occur during the early spring or late 778 autumn season (before or after the growth cessation; Fig. 11b) supports this hypothesis. Spring or autumn

788
The occurrence of spring blooms is supported by weak 0.5-1.0‰ seasonal variability in  13 C (Fig. 6).

789
Seasonal changes in productivity and/or salinity will cause changes in DIC in the environment, which are 790 apparent in the  13 C of the shell above the ontogenetic trends and the effect of respiration on  13 C (see 2.4; 791 Table 1). The fact that a clear seasonality in  13 C is absent from the stack in Fig. 9 shows that these 792 productivity peaks do not occur at regular times of the year and that their effect on  13 C is obscured by 793 ontogenetic trends. The 0.5-1.0‰ shifts in  13 C that appear to be seasonal are much smaller than those in

859
Finally, the coupled modelling and multi-proxy approach applied in this study sheds light on the effects of 860 environmental changes on the life cycle and sub-annual growth of R. diluvianum shells. This study reveals 861 that growth curves of R. diluvianum strongly resemble those in modern shallow marine bivalves that grow 862 in coastal high latitude environments. However, ontogenetic changes in growth rate of our Boreal oysters 863 seem unrelated to temperature, in contrast to modern, high-latitude oysters that tend to lower their growth 864 rate and cease mineralization below a certain cold threshold. We conclude that growth cessations and sub-865 annual changes in growth rate in R. diluvianum were most likely not caused by intolerable temperatures, 866 but rather by circadian rhythm tied to the seasonal cycle and seasonal changes in sea surface productivity, 867 driven by nutrient-rich freshwater inputs.