Size-normalized Weight Research Articles

Influences of Atlantic Ocean thermohaline circulation and Antarctic ice-sheet expansion on Pliocene deep Pacific carbonate chemistry

Quantifying changes in seawater carbonate chemistry is crucial to deciphering of patterns and drivers of the oceanic carbon cycle and climate change. Here, we present a new deep-water carbonate ion saturation state (Δ[CO32−]) record for the Pliocene western tropical Pacific, reconstructed from the size-normalized weight of the planktonic foraminifer Trilobatus sacculifer of IODP Site U1490. A steep decline in deep-water Δ[CO32−] occurred at ∼4.6 Ma synchronous to the enhanced production of North Atlantic Deep Water (NADW) related to the closure of the Panamanian Gateway. Subsequently, at the onset of the Northern Hemisphere glaciation at ∼2.7 Ma the weakening of NADW formation resulted in a deep-water Δ[CO32−] peak. The changes in NADW production rate likely controlled a seesaw-like fluctuation in deep-water Δ[CO32−] between the Pacific and Atlantic oceans. During the late Pliocene (∼3.8–2.8 Ma), Antarctic ice-sheet/sea-ice expansions sequestered CO2 in the deep Pacific through ventilation of the deep watermass, leading to a long-term decrease in deep Pacific Δ[CO32−]. We infer that fluctuating NADW production rates at ∼4.6 Ma and ∼2.7 Ma influenced inter-basinal fractionation of deep-ocean carbon between the Atlantic and Pacific, and that deep Pacific carbon storage linked to expansions of Antarctic ice sheet/sea ice contributed to the lowering of atmospheric pCO2 and global cooling during the late Pliocene.

Response of Planktic Foraminiferal Shells to Ocean Acidification and Global Warming Assessed Using Micro-X-Ray Computed Tomography

Ocean acidification is now progressing, primarily due to the fact that the oceans have absorbed about 50% of the anthropogenic CO2 emitted since the industrial revolution. Many marine calcifying organisms, such as foraminifers and coccoliths, are known to build their shells using carbonate ions present in the seawaters surrounding them. Carbonate saturation state has a crucial influence on foraminiferal calcification, and foraminiferal shell production is known to be sensitive to increase in ocean pCO2. Moreover, ocean warming is also progressing along with acidification. Therefore, both environmental changes could affect foraminiferal shell formation. However, the relationship between foraminiferal shell parameters (i.e., size, weight, volume, and density) and ocean pCO2 or sea surface temperature (SST), or both, remains unclear. In this study, we used fossil planktic foraminifer Globigerinoides ruber (white) in a late Quaternary sediment core (MD98-2196) from the East China Sea to investigate a relationship between the shell parameters and oceanographic properties estimated based on the proxies from the same core. The foraminiferal shells were scanned using high-resolution micro-X-ray computed tomography (MXCT) to determine shell volume and density. The results showed that the size-normalized weight and the size-normalized volume of the shell had a negative correlation with the SST and atmospheric pCO2. The negative correlation between weight/volume and atmospheric pCO2 agrees with the previous laboratory experiments and geological record during the Pliocene. However, the correlation between weight/volume and SST should be interpreted with caution because it might be an artifact due to the correlation between SST and atmospheric pCO2. On the other hand, shell density is only weakly or insignificantly correlated with SST and pCO2, suggesting that these environmental parameters do not exert any impact on shell density. Thus, future ocean acidification will negatively affect the carbonate productivity of planktic foraminifers, even if it will not affect shell density. The temperature effect on the shell formation of the planktic foraminifers might be much less than ocean acidification considering controversial results of the temperature sensitivity in previous studies.

Calcification of planktonic foraminifer Pulleniatina obliquiloculata controlled by seawater temperature rather than ocean acidification

Abstract Planktonic foraminifera represent a major component of global marine carbonate production, and understanding environmental influences on their calcification is critical to predicting marine carbon cycle responses to modern climate change. The present study investigated the effects of different environmental influences on calcification of the planktonic foraminifer Pulleniatina obliquiloculata. By correcting the dissolution effect on the size-normalized weight (SNW) of P. obliquiloculata from deep-sea sediments, we provide a means of estimating initial size-normalized weight (ISNW) from which to assess secular changes in the degree of calcification of P. obliquiloculata. Core-top ISNW in P. obliquiloculata from the global tropical oceans is significantly positively correlated with calcification temperature, suggesting that temperature is the dominant control on calcification. Using Neogloboquadrina dutertrei SNW as an independent deep-water Δ[CO32−] proxy, we present an ISNW record for P. obliquiloculata from the western tropical Pacific since 250 ka. The response of ISNW to past seawater temperature variations further confirms the dominant influence of temperature on P. obliquiloculata calcification. A potential increase in calcification as a result of ocean warming may have reduced oceanic uptake of CO2 from the atmosphere and increased atmospheric pCO2, generating a positive feedback for global warming.

Planktic Foraminiferal Test Size and Weight Response to the Late Pliocene Environment

AbstractAtmospheric carbon dioxide (pCO2atm) is impacting the ocean and marine organisms directly via changes in carbonate chemistry and indirectly via a range of changes in physical parameters most dominantly temperature. To assess potential impacts of climate change on carbonate production in the open ocean, we measured size and weight of planktic foraminifers during the late Pliocene at pCO2atm concentrations comparable to today and global temperatures 2 to 3 °C warmer. Size of all foraminifers was measured at Atlantic Ocean Deep Sea Drilling Project (DSDP) Site 610, Ocean Drilling Program (ODP) Site 999, and Integrated Ocean Drilling Program (IODP) Site U1313. Test size was smaller during the Pliocene than in modern assemblages under the same environmental conditions. During the cold marine isotope stage (MIS) M2, size increased at Site 999, potentially linked to intensified stratification of the surface ocean in response to the closure of the Central American Seaway. At Site U1313, test size tracks the warming throughout the late Pliocene. Size‐normalized weight (SNW) of Globigerina bulloides at Site U1313 decreased during warmer temperature intervals. SNW of Globigerinoides ruber (white) at Site 999 displays high‐frequency variability not correlated to temperature. Yet during the glacial period within MIS M2, test weight was higher during higher temperatures. Our results support studies in the modern ocean, which challenge the view that carbonate chemistry is the primary driver for calcification. To better understand processes driving changes in SNW, computer tomography was used to quantify calcite to volume ratios. During interglacial periods, lower calcite volume but higher test volume suggests less suitable conditions for calcification. As this signal is not evident in SNW, subtle changes in calcification might not be observed by the weight‐based method.

Coccolith morphological and assemblage responses to dissolution in the recent sediments of the East China Sea

Abstract Evaluating carbonate dissolution in deep sea sediments is of key importance in understanding the variation of the carbonate compensation depth and the ocean carbon cycle in the geological past. Since coccoliths are one of the main contributors to oceanic CaCO3, their dissolution and preservation degrees in sediments can be a useful indicator for deep sea carbonate chemistry. Varying coccolith preservation conditions have been found due to dissolution caused by organic matter degradation in the recent surface sediments of the East China Sea, which provides a good basis for the study of coccolith morphological and assemblage responses to dissolution. We measured the coccolith weight, thickness, and length of Gephyrocapsa spp. (>3 μm) using a circularly polarized light microscope. It has been found that Gephyrocapsa spp. (>3 μm) coccoliths become thinner and lighter in response to dissolution, and coccolith assemblages are also altered in poorly preserved sediments. This phenomenon was confirmed by an acidification experiment on a sediment sample, which also showed that coccoliths became thinner and lighter under increasingly acidified conditions. There is selective dissolution, i.e., Emiliania huxleyi coccoliths are most dissolution-prone, followed by Gephyrocapsa spp. ( 3 μm), and Helicosphaera spp.. Coccolith morphological parameters can be used to quantitatively evaluate coccolith preservation and dissolution in sediment samples. We suggest that using size-normalized weight, a mean coccolith weight loss of ~30–50% can be assigned to moderate-poor preservation for coccoliths, as reflected by the measured coccolith morphological changes in the surface sediments and in the acidification experiment.

Benthic foraminiferal shell weight: Deglacial species-specific responses from the Santa Barbara Basin

Abstract Here we present a record of size-normalized shell weight for four species of benthic foraminifera through a period of rapid environmental change during the most recent deglaciation (Santa Barbara Basin, CA). A strong Oxygen Minimum Zone (OMZ), the product of high surface productivity and poor ventilation, characterizes the eastern Pacific; this subsurface zone is mechanistically coupled with high concentrations of dissolved inorganic carbon. The OMZ migrated vertically during warming of the last deglaciation, leading to rapid shifts in the oxygenation and inorganic carbon system of the benthos. The size-normalized weight (SNW) of benthic foraminifers Uvigerina peregrina , Bolivina interjuncta , and Bolivina tumida reflects only the broad trends of the vertical migration of the OMZ, and inorganic carbon system, overshadowed by clear species-specific trends. The relative importance of OMZ migrations versus other environmental variables and optimal growth conditions differs across species of benthic foraminifera. In U. peregrina , SNW primarily peaks with foraminiferal density and increased abundance of that species, while B. interjuncta and B. tumida increase in SNW with a shrinking of the OMZ (and carbon maximum) in the late Holocene. Bolivina argentea shows no long-term trends in SNW potentially due to its ability to migrate through the sediment. Our results suggest that, while inorganic carbon and dissolved oxygen may play a role in determining shell weight across species of benthic foraminifera, neither parameter alone is responsible for changes in benthic foraminiferal shell weight in the fossil record.

Seasonal Variation in Shell Calcification of Planktonic Foraminifera in the NE Atlantic Reveals Species-Specific Response to Temperature, Productivity, and Optimum Growth Conditions.

Using shells collected from a sediment trap series in the Madeira Basin, we investigate the effects of seasonal variation of temperature, productivity, and optimum growth conditions on calcification in three species of planktonic Foraminifera. The series covers an entire seasonal cycle and reflects conditions at the edge of the distribution of the studied species, manifesting more suitable growth conditions during different parts of the year. The seasonal variation in seawater carbonate saturation at the studied site is negligible compared to other oceanic regions, allowing us to assess the effect of parameters other than carbonate saturation. Shell calcification is quantified using weight and size of individual shells. The size–weight scaling within each species is robust against changes in environmental parameters, but differs among species. An analysis of the variation in calcification intensity (size-normalized weight) reveals species-specific response patterns. In Globigerinoides ruber (white) and Globigerinoides elongatus, calcification intensity is correlated with temperature (positive) and productivity (negative), whilst in Globigerina bulloides no environmental forcing is observed. The size–weight scaling, calcification intensity, and response of calcification intensity to environmental change differed between G. ruber (white) and G. elongatus, implying that patterns extracted from pooled analyses of these species may reflect their changing proportions in the samples. Using shell flux as a measure of optimum growth conditions, we observe significant positive correlation with calcification intensity in G. elongatus, but negative correlation in G. bulloides. The lack of a consistent response of calcification intensity to optimum growth conditions is mirrored by the results of shell size analyses. We conclude that calcification intensity in planktonic Foraminifera is affected by factors other than carbonate saturation. These factors include temperature, productivity, and optimum growth conditions, but the strength and sign of the relationships differ among species, potentially complicating interpretations of calcification data from the fossil record.

Patterns of European vegetation changes over the last 15,000 years based on the European Pollen Database with new chronologies

Quantifying changes in seawater carbonate chemistry is crucial to deciphering of patterns and drivers of the oceanic carbon cycle and climate change. Here, we present a new deep-water carbonate ion saturation state (Δ[CO32−]) record for the Pliocene western tropical Pacific, reconstructed from the size-normalized weight of the planktonic foraminifer Trilobatus sacculifer of IODP Site U1490. A steep decline in deep-water Δ[CO32−] occurred at ∼4.6 Ma synchronous to the enhanced production of North Atlantic Deep Water (NADW) related to the closure of the Panamanian Gateway. Subsequently, at the onset of the Northern Hemisphere glaciation at ∼2.7 Ma the weakening of NADW formation resulted in a deep-water Δ[CO32−] peak. The changes in NADW production rate likely controlled a seesaw-like fluctuation in deep-water Δ[CO32−] between the Pacific and Atlantic oceans. During the late Pliocene (∼3.8–2.8 Ma), Antarctic ice-sheet/sea-ice expansions sequestered CO2 in the deep Pacific through ventilation of the deep watermass, leading to a long-term decrease in deep Pacific Δ[CO32−]. We infer that fluctuating NADW production rates at ∼4.6 Ma and ∼2.7 Ma influenced inter-basinal fractionation of deep-ocean carbon between the Atlantic and Pacific, and that deep Pacific carbon storage linked to expansions of Antarctic ice sheet/sea ice contributed to the lowering of atmospheric pCO2 and global cooling during the late Pliocene.

Testing planktic foraminiferal shell weight as a surface water [CO32−] proxy using plankton net samples

Planktic foraminiferal size-normalized weight (SNW) has been used as a proxy for both past changes in deepwater dissolution and surface ocean [CO32−], the latter potentially providing a way to evaluate paleoatmospheric p CO2 variations beyond the ice core records. Here we examine the relationship between SNW in modern planktic foraminifera and surface water [CO32−] in the Arabian Sea using a suite of samples obtained from plankton net casts in surface waters having a large range in their carbon chemistry. Our results reveal substantial interspecies- and intraspecies-specific variations in the strength, gradient, and even sign of this relationship, indicating that [CO32−] does not exert a dominant control on foraminiferal test weight. Similarly, foraminiferal abundance data do not lend support to the hypothesis that SNW responds to optimal growth conditions. Further work is needed, perhaps in laboratory cultures, to determine those environmental factors that are simply correlated with SNW and those that exert control.

A novel application of wet oxidation to retrieve carbonates from large organic‐rich samples for ocean‐climate research

For ocean‐climate research, carbonates are extracted from large organic‐rich bulk sediments for specific geochemical analyses. This is conventionally achieved by time‐consuming dry oxidation or nonoxidative preprocessing. To significantly shorten sample preparation time we designed and evaluated a rapid technique that uses sequential wet oxidization of bulk samples in a hot alkaline 18% H2O2 solution. We successfully tested this wet oxidation technique on multiple bulk aliquots from two sediment trap samples and a core top sediment that were also processed by dry oxidation and without oxidation. From all aliquots four calcitic foraminifera species and an aragonitic juvenile bivalve from the 250–315 μm fraction were analyzed for size‐normalized weight (SNW), δ18O, δ13C, Mg/Ca, and Sr/Ca. In addition, these proxies were determined on powdered bulk aliquots from the 150–250 μm fraction processed by wet oxidation, dry oxidation, and without oxidation. At an initial pH of 8 and temperature of 70°C the alkaline H2O2 solution appeared to be most stable and reactive. Carbonate dissolution did not occur as no reduction was observed in SNW of the four foraminifera species G. ruber, G. trilobus, N. dutertrei, G. bulloides, and the aragonitic bivalve shells. For sediment traps the δ18O and δ13C between the three cleaning methods only deviated between 0.05 and 0.3‰ compared to 0.1 and 0.6‰ in sediments, with equally small variation in Mg/Ca and Sr/Ca ratios (0.1–0.4 mmol/mol). Lowest Ba/Ca ratios show that wet oxidation successfully removed organic matter along with residual salts. No significant systematic differences between samples or methods were found, but a residual scatter remained, particularly in the nonoxidized sediment because of intraspecies and intrashell inhomogeneity. Given proper preprocessing all three techniques performed well, with the proposed wet oxidation method emerging as a fast technique for extracting carbonate shells from wet, gram‐sized bulk samples. Within 3 h wet oxidation produced clean, dry residues of unaltered calcareous shells from batches of four wet bulk samples without time‐consuming intermediate steps. For Mg/Ca thermometry on time series sediment trap samples, wet oxidation results approached those obtained using the Barker et al. (2003) protocol with improved sample recovery. Consequently, the proposed wet oxidation offers a rapid alternative to conventional extraction techniques for carbonate geochemistry.