Marine plankton is an important component of the global carbon cycle. Whereas the production and seafloor export of organic carbon produced by the plankton, the biological pump, has received much attention, the long-term variability in plankton calcification, controlling the carbonate counter pump, remains less well understood. However, it has been shown that changes in pelagic calcification (biological compensation) could affect the ocean's buffering capacity and thus regulate global carbon budget on geological timescales. Here we use Neogene pelagic sediments deposited on the Ceara Rise in the tropical Atlantic to characterize the variability in pelagic carbonate production with a focus on warm climates. A re-evaluation of published records of carbonate accumulation at the Ceara Rise reveals a systematic increase in sedimentation rates since the late Miocene, but the carbonate accumulation rate does not show a clear trend. Instead, we observe substantial orbital timescale variability in carbonate accumulation, combined with a trend towards less carbonate on average at sites located below 4 km, likely due to the effect of carbonate dissolution. To evaluate long-term changes against possible orbital-scale variability, we generated new high-resolution records of carbonate accumulation rate at Ocean Drilling Program (ODP) Site 927 across two Quaternary interglacials (MIS 5 and MIS 9), the Pliocene warm period (MIS KM5) and the Miocene Climatic Optimum (MCO). We observe that the highest carbonate accumulation rates occurred during the Pliocene but that each of the studied intervals was characterized by large-magnitude orbital variability. Prominent variations in carbonate accumulation prior to the Quaternary preservation cycles appear to follow Earth obliquity and eccentricity. These results imply that pelagic carbonate accumulation in the tropical ocean, buffered from large temperature changes, varied on orbital timescales. The magnitude of the orbital-scale variability was similar or even higher than the long-term mean differences among the studied intervals. Since preservation can be excluded as a driver of these changes prior to the Quaternary, the observed variations must reflect changes in the export flux of pelagic biogenic carbonate. We conclude that the overall carbonate production by pelagic calcifiers responded to local changes in light, temperature, and nutrients delivered by upwelling, which followed long orbital cycles, as well as to long-term shifts in climate and/or ocean chemistry. The inferred changes on both timescales were sufficiently large such that when extrapolated on a global scale, they could have played a role in the regulation of the carbon cycle and global climate evolution during the transition from the Miocene warm climates into the Quaternary icehouse.
The ocean plays a key role in the climate system as one of the major sinks
for anthropogenic atmospheric CO
For the process of biological compensation to play an important role in the global carbon cycle, it must be demonstrated that sufficiently large changes in global carbonate biomineralization occurred in the geological past. However, measuring changes in global biogenic carbonate production is difficult because productivity and biomineralization vary in space, and changes observed in individual records could be compensated by complementary shifts elsewhere in the ocean (Drury et al., 2021). In most parts of the ocean, climate change causes plankton assemblages to migrate, with biogeographic provinces expanding and contracting in pace with orbital cycles (Yasuhara et al., 2020). These processes should result mainly in the spatial reorganization of pelagic carbonate production, and as long as the forcing is cyclic, the effects should cancel out over time.
Beyond orbital timescales, understanding of changes in carbonate production are complicated by the confounding effects of biological and chemical compensation on carbonate content of deep-sea sediments (Boudreau et al., 2018). Nevertheless, the few existing continuous records indicate the presence of long-term shifts in carbonate production by a factor of 2 or more manifested, for example, as the late Miocene carbonate maximum (Lyle et al., 2019; Drury et al., 2021; Liebrand et al., 2016). Although there is abundant evidence for local changes in pelagic calcification and carbonate production, their spatial extent remains unknown, making it difficult to judge whether the local shifts may have resulted in globally significant biogeochemical response (Lyle et al., 2019; Drury et al., 2021).
Here we have investigated pelagic carbonate accumulation, as a proxy for
production, in an equatorial location, where the plankton could not respond
to the climate cycles by migration and where long-term changes in
temperature, a key parameter likely affecting biomineralization, were
buffered compared to higher latitudes. Low-magnitude tropical sea surface temperature (SST)
variability in the Atlantic in the Pliocene and in the Miocene was reported
by Herbert et al. (2016) and Curry et al. (1995). Since orbitally driven environmental
change still affected the tropics, the Cenozoic tropical plankton represents
a natural experiment where the tropical calcifying community responded to a
number of orbital cycles and long-term changes in ocean chemistry,
reflecting changing atmospheric CO
Next to analysing long-term changes in carbonate accumulation, the existence of persistent orbital variability implies that new data will be required, characterizing the short-term response of the tropical pelagic carbonate production system. To this end, in the present study the changes in carbonate production through time have been studied in four intervals, occurring during four warm periods of the late Cenozoic: the marine isotopic stage (MIS) 5 (87.5 to 150.2 ka), the MIS 9 (276.4 to 370.3 ka), the MIS KM5 (3095.5 to 3307 ka) and the Miocene Climatic Optimum (MCO) (15 589.3 to 15 964.3 ka).
This approach allows us to evaluate long-term changes in pelagic carbonate production since the Mid-Miocene and at the same time to characterize the orbital-scale variability and determine if the orbital periodicity forcing carbonate production changed from the Miocene to present.
The MIS 5, as the last warmest and longest interglacial of the past 500 ka
(Howard, 1997), with an abrupt glacial–interglacial transition
(Howard, 1997; Müller and Kukla,
2004; Sirocko et al., 2005) is considered to be a good analogue for the
actual warm Holocene (Howard, 1997; Kukla, 1997) and
even a partial analogue for
The MIS 9 in the equatorial Atlantic presents well-preserved sediment
at a period known to be under high obliquity with a unique insolation
signal. Stable oxygen isotope values are low during this period (low ice
volume). It is one of the interglacials showing the highest
The Pliocene warm period (PWP) MIS KM5 corresponds to a period with a
similar orbital forcing to present day and an insolation distribution close
to the modern one (Haywood et al.,
2013). This interval (3.264–3.025 ka) is also described as a negative
oxygen isotope slope and a sea level 21–23 m above the present-day one
(Lunt
et al., 2008, 2010; Naish et al., 2009; Pollard and DeConto, 2009) with a
well-ventilated deep Atlantic Ocean
(Bell et al., 2015). The
temperature is 3
The MCO corresponds to a period with an eccentricity-modulated precession
Location of the material of this study at the Ceara Rise, ODP Leg 154 (Ocean Data View, Schlitzer, 2018).
Ceara Rise, located in the equatorial Atlantic Ocean, represents an ideal location to quantify the variability in tropical Atlantic pelagic carbonate production since the Miocene. This aseismic ridge rises several kilometres above the surrounding abyssal plain, well above the modern regional lysocline, located between 4100 and 4200 m b.s.l. (Frenz et al., 2006; Gröger et al., 2003a, b; Curry et al., 1995; Cullen and Curry, 1997; Bickert et al., 1997). The ridge is bathed by the shallower North Atlantic deep water (NADW) and the deeper Antarctic bottom water (AABW) (Rühlemann et al., 2001; Gröger et al., 2003b; Herrford et al., 2017), and the interface of the two water masses corresponds to the regional lysocline depth. Around the ridge, the average depth of the seafloor is at 4500 m b.s.l., but the Ceara Rise ridge rises by as much as 1900 m above the surrounding abyssal plain, with its top reaching the depth of 2600 m b.s.l. (Curry et al., 1995). This provides an opportunity to sample pelagic sediments that are largely unaffected by dissolution, and their accumulation therefore mainly reflects changes in pelagic carbonate production as suggested by Brummer and van Eijden (1992). The Ceara Rise (Fig. 1) has been visited by Ocean Drilling Program (ODP) Leg 154 (Curry et al., 1995), recovering a transect of sediment sequences ranging into the Eocene that are rich in carbonate and show prominent cycles due to variable input of clastic material from the Amazon fan (Shackleton et al., 1999; Bickert et al., 1997; Shackleton and Crowhurst, 1997). The cycles are reflected in sediment physical properties, such as colour or magnetic susceptibility, and because of the very good recovery and repeated coring at the same sites, continuous spliced records could be produced that facilitated the development of orbitally tuned age models (Shackleton et al., 1999; Zeeden et al., 2013; Wilkens et al., 2017; Shackleton and Crowhurst, 1997), a prerequisite for the quantification of carbonate accumulation. Since all high-resolution Neogene records of carbonate accumulation (Drury et al., 2021; Lyle et al., 2019), including those from the Ceara Rise (Curry et al., 1995; King et al., 1997) show a large orbital-scale variability, hinting at prominent orbital-scale variability in pelagic carbonate production, next to a compilation and re-evaluation of existing carbonate records, the selected time slices had to be newly sampled and analysed at higher resolution.
The combination of the availability of high-resolution age models and good
carbonate preservation make the Ceara Rise a model region to study pelagic
carbonate production and preservation. We compiled existing data on
carbonate content (CaCO
The carbonate content data were combined with dry bulk density (DBD) and
sedimentation rate (SR) to calculate the CaCO
Oxygen stable isotopes (
We sampled the record at Site 927 at high resolution for the four periods of
interest (Fig. 2), making sure that for each interval both the interglacial
and the flanking glacial in the Quaternary and at least two full
eccentricity cycles during the Pliocene and Miocene have been covered. These
four intervals cover a large range of global temperature and CO
We performed stable isotopes analyses (
Because the orbitally tuned age models as well as the splices for the individual sites have been recently revised (Wilkens et al., 2017), we re-evaluated the composite depth of all samples and assigned new ages to them based on Wilkens et al. (2017) and used the new ages to derive sedimentation rates (SR).
The existing most recent age model for Site 927 is based on a directly tuned
age model from Site 926 that has been point-to-point correlated with the
composite record from Site 927 using core images, magnetic susceptibility,
greyscale values and stable isotopes
(Wilkens et al.,
2017; Zeeden et al., 2013). For the determination of CaCO
To determine the CaCO
Using existing carbonate content data for all Leg 154 sites
(Curry
et al., 1995; Frenz et al., 2006; King et al., 1997), combined with new age
models (Wilkens et al., 2017), for each
site, records of CaCO
The mean CaCO
Depth–age correlation for the late Pleistocene, cores 927A 1H,
927B 2H and 927A 2H (following the splice) with
The Pleistocene interval in the studied core has a high-resolution age model
based on benthic oxygen isotope data (Bickert et al., 2004)
that were incorporated in the benthic stack of Lisiecki and Raymo (2005), who had added a constant 4–5 kyr lag to take into account the
delay in the
Depth–age correlation for the Pliocene interval across cores 154
927C 11H, 154 927A 12H and 154 927B 13H.
For the Pliocene interval, the first step has been to validate the core
alignment. First, we generated a grey value curve (Sect. 3.2.1) but noted
that this signal is weaker and shows many idiosyncratic features among the
overlapping parts of the cores from the individual holes. Therefore, we
decided to carry out the tuning on the magnetic susceptibility (MS) signal
as done by Shackleton et al. (1999), which was also measured
in all cores (Curry et al., 1995). MS shows a distinct signal in this part
of the sediment sequence, which can be used for tuning (like it has been
used at Site 926), but for this it must be in alignment across the
individual core segments. The alignment revealed that the existing splice by
Wilkens et al. (2017) has to be adjusted for the purpose of tuning in this
interval (Fig. S2) by a shift of the core 927C 11H by 2 cm shallower, a
shift of the core A 12H by 15 cm deeper and a shift of the core B 13H 9 cm deeper in the splice compared to the spliced MS record of Wilkens et al. (2017). Otherwise, the construction of the spliced record remained the same,
retaining the same depths where the signal from one core switches to a
signal from the adjacent core. These depths are indicated by dashed lines
across the overlapping sections of the cores (Fig. 6d). The spliced MS
signal (Fig. 6b) has then been tuned to the daily insolation on 21 June
at 65
Depth–age correlation for the Mid-Miocene, core 154 927 A33H.
The existing age model for the Miocene interval by Shackleton et al. (1999)
is based on a combination of orbital tuning and biostratigraphy. It presents
a distinct shift in the SR around 330 mcd (Fig. 7c), dominating the
CaCO
To have an independent estimation of the SR, we also evaluated the
biostratigraphy from the shipboard data (Curry et al., 1995) with revised metre composite depth
(Wilkens et al., 2017) and revised biomarker ages GTS 2020
(Raffi et al., 2020). Three biostratigraphic
markers have been evaluated: last appearance datum (LAD) of
A sediment colour proxy was generated for the studied core (Sect. 3.2.1)
(Fig. 7d). Due to the light appearance of the sediment composing this core
and the way the pictures have been taken onboard (1.5 m sections with a centred camera and centred white source of
light), there is a strong 1.5 m induced light cyclicity in the original
light images (Curry et al., 1995; Wilkens et al., 2017). To reduce this
bias, the core images were adjusted for the edge effect using the lighting
correction function inside the Code for Ocean Drilling Data (CODD, Wilkens
et al., 2017) (Fig. 7d). For the identification of the cyclicity in the
core, we carried out spectral analyses on the corrected grey value curve
using the multitaper method (MTM) (carried out using astrochron package on
R, Meyers, 2014; R 4.1.2., R Core Team, 2021)
(Fig. S6). This revealed three broad but distinct peaks for the frequencies
0.48 (period: 2.08 m), 0.7 (period: 1.43 m) and 1.4 (period: 0.71 m).
Applying the two alternatives, biostratigraphy-derived SR reveals that the
most distinct 71 cm cycles could represent obliquity when the SR of 1.65 cm kyr
The new carbonate content analyses are based on 261 measurements, yielding
values comparable to existing low-resolution measurements, confirming
decreasing carbonate content throughout the Neogene due to dilution by
clastic sediments from Amazon fan
(Curry et al., 1995; Bickert
et al., 1997; Harris et al., 1997) and indicating particularly strong
variations in the Quaternary (Fig. 8). In combination with the new
high-resolution SR data (Fig. 8), these measurements provide records of
sub-orbital variability in CaCO
The comparison between the highly resolved record for the four intervals of
interest (Fig. 8c, d, and e) and the environmental parameters (Fig. 8a and
b) highlights the good correlation – in terms of phase and amplitude –
between the CaCO
The presence of multiple CaCO
CaCO
When we look at the trend of the highest values reached on long geological
timescale from the mid-Miocene to MIS 5 (Fig. 10), we observe a 31 %
decrease in CaCO
Looking at the amplitude of the variability within the Pliocene and Miocene
interval, we found higher values in the Pliocene (65 %) compared to the
average of the period (1.98 g cm
During the Pleistocene, the CaCO
Because the Ceara Rise sites became periodically affected by the more
corrosive Antarctic bottom water only after the initiation of the North
Hemisphere glaciation
(Liebrand
et al., 2016; Harris et al., 1997; Pälike et al., 2006a), the studied
Pliocene and Miocene intervals should not be affected by dissolution. Paul
et al. (2000) note that the exact subsidence history of the
Ceara Rise is unknown but assume minimal subsidence since the early Miocene.
Similarly, sea level differences among Quaternary interglacials and the Pliocene
and Miocene were likely on the order of tens of metres. Therefore, the
largest changes in palaeodepth would have been due to sediment cover, which
would make the studied mid-Miocene interval about 300 m deeper compared to
the present one (this depth is still above the present day lysocline depth
of 4200 m b.s.l.). Throughout the entire studied interval since the Miocene
(Fig. 3), the shallowest cores (925 and 927) record higher CaCO
MTM spectral analysis of the CaCO
Assuming dissolution did not play a significant role in the observed
variations in CaCO
Coherence diagram BT cross correlation (Paillard et al., 1996)
between the CaCO
Comparison of the CaCO
Comparison of the CaCO
Finally, we consider the apparent shift in the phase in the relationship
between orbital forcing and the CaCO
Because of the observed changes in what appears to be carbonate production among the studied intervals and especially within the studied intervals, we conclude that tropical pelagic calcifiers responded to environmental or biotic forcing on orbital cycles, as well as to long-term shifts in climate and/or ocean chemistry. In other words, either the production, the community composition or the biomineralization of the tropical pelagic calcifiers may respond to local changes in light, temperature and nutrients delivered by upwelling, which followed orbital cycles, as well as to long-term shifts in climate and/or ocean chemistry. The inferred changes in pelagic carbonate production on both timescales are sufficiently large that when extrapolated on a global scale, they could have played a role in the regulation of the carbon cycle. For example, Boudreau et al. (2018) estimated that changes in global pelagic carbonate production on the order of 10 % would be sufficient to affect the marine carbon cycle on timescales from years to millions of years. Whereas the drivers of the orbital-scale variability could be plausibly attributed to changes in local oceanic parameters affecting primary production, the causes of the long-term shifts require another explanation.
There are two studies presenting long continuous CaCO
A compilation of CaCO
To analyse long-term and orbital-scale patterns of pelagic carbonate production variability, we generated new data for four periods at Site 927.
We found that CaCO
We conclude that the low-latitude pelagic carbonate production responded
strongly to orbital-driven local tropical processes, rather than to secular
changes in the global climate or ocean chemistry (like global CO
Our results imply that in the context of the ongoing and projected global change, pelagic carbonate production may be an important variable in the parameterization of the global marine carbon cycle, especially with regard to the long-term (millennial-scale) fate of anthropogenic carbon injection. To parameterize the pelagic carbonate production, it remains to be shown whether it changes due to changes in production (population sizes), biomineralization (amount of carbonate produced per individual) or community composition (shift to more or less calcified taxa).
All data sets are available on Pangaea (
The supplement related to this article is available online at:
The conceptualisation, as well as samples selection, was carried out by all the co-authors. PC compiled all the existing carbonate data for he five sites of Leg 154 and calculated carbonate accumulation rates from it. PC analysed all of the newly generated carbon content from the samples of this study, calculated the carbonate content and accumulation rates from it, and revised the tuned age models for the three periods of interest in this study (with the help of TW and MK). PC ran the spectral analysis and prepared all the figures presented in this paper. PC wrote the manuscript, and all co-authors contributed to the manuscript.
The contact author has declared that none of the authors has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank Brit Kockisch for assistance with carbonate content analyses and Anna-Joy Drury for providing South Atlantic carbonate data and discussing the results.
This research used samples and data provided by the Ocean Drilling Program (ODP), which is sponsored by the US National Science 15 Foundation (NSF) and participating countries. This research was supported by the DFG through Germany's Excellence Strategy, Cluster of Excellence “The Ocean Floor – Earth's Uncharted Interface” (EXC-2077, Project 390741603). The article processing charges for this open-access publication were covered by the University of Bremen.
This paper was edited by Caroline P. Slomp and reviewed by two anonymous referees.