4.3. Practical implications for coral-based SST reconstructions
Most if not all fossil corals are subject to some level of diagenetic
alteration, motivating the development of an evidence-based protocol for
acquiring accurate paleotemperature estimates from this archive. This is
especially true because, except in the most extreme cases, diagenesis
can escape detection via standard screening protocols. While alteration
is often associated with older fossil corals, diagenesis routinely
occurs in relatively young corals, such as the <100yr-old
fossil coral featured in this study, and is also observed in cores
collected from living colonies (e.g. Nothdurft et al. , 2007;Nothdurft and Webb , 2008; Rabier et al. , 2008;Nurhati et al. , 2011; Sayani et al. , 2011). As the
composition of secondary cements is significantly different from that of
coral aragonite, even light to moderate levels of alteration can produce
significant errors reconstructions that rely on bulk analytical
techniques. Moreover, given the scarcity of fossil corals, researchers
often attempt to recover reconstructions from corals that grew during a
given time period of interest, even if they have experienced moderate to
significant alteration. Thus, given the pervasiveness of diagenesis and
its substantial impact on accuracy of fossil coral Sr/Ca-based
paleotemperature estimates, rigorous screening for diagenesis would
ideally be combined with micro-scale SIMS Sr/Ca as an independent
constraint on paleotemperature.
SIMS Sr/Ca of coral skeletons allows for the selective analysis of less
altered skeletal material, but the cost- and labor-intensive nature of
these analyses requires a sampling strategy that maximizes the accuracy
of the resulting paleotemperature reconstructions with as few analyses
as possible. Our results suggest that the most strategic use of SIMS
Sr/Ca involves analysis of discrete multi-year windows to check the
accuracy of longer bulk coral Sr/Ca timeseries generated by ICP-OES. In
doing so, it is important to consider how to achieve the following goals
with a minimum number of SIMS analyses: collecting enough measurements
in a given time interval to overcome microscale compositional
heterogeneity in coral aragonite, and collecting timeseries of SIMS
measurements that will provide an apples-to-apples comparison to bulk
Sr/Ca measurements. By sampling ~3-4 samples per month
of coral growth, and applying a ~2.5 month running mean
filter, we obtained monthly-resolved Sr/Ca with an analytical
uncertainty of ±0.04mmol/mol or ±0.6˚C (1σ). While this is adequate for
validating bulk Sr/Ca, it may not be sufficient for exploring
paleotemperature changes without accompanying longer, bulk Sr/Ca
timeseries. Increasing the number of SIMS Sr/Ca analyses per month of
coral growth would indeed reduce the analytical uncertainty of
SIMS-based Sr/Ca timeseries. For example, ~20
analyses/month would yield uncertainties as low as ±0.022mmol/mol
(1sigma) or ±0.3˚C in smoothed SIMS Sr/Ca, assuming a Gaussian
distribution, however this may not be practical in most cases.
Additionally, paleo-SST reconstructions from fossil corals must also
contend with intercolony offsets, such as the ±0.08mmol/mol or ±1˚C (1σ)
offset observed in SIMS data from the three modern corals studied here,
that make it difficult attribute differences in mean Sr/Ca values
between two corals to climate change (e.g. Sayani et al. , 2019).
As such, given that multiple corals are needed from a desired time
interval to derive robust estimates of mean paleo-SST change, the most
practical use of SIMS coral Sr/Ca is to validate the accuracy of bulk
coral Sr/Ca timeseries.