Environmental Predictors of Diversity in Recent Planktonic Foraminifera as Recorded in Marine Sediments

Biogeographic response of marine plankton to Cenozoic environmental changes

In palaeontological studies, groups with consistent ecological and morphological traits across a clade's history (functional groups)1 afford different perspectives on biodiversity dynamics than do species and genera2,3, which are evolutionarily ephemeral. Here we analyse Triton, a global dataset of Cenozoic macroperforate planktonic foraminiferal occurrences4, to contextualize changes in latitudinal equitability gradients1, functional diversity, palaeolatitudinal specialization and community equitability. We identify: global morphological communities becoming less specialized preceding the richness increase after the Cretaceous-Palaeogene extinction; ecological specialization during the Early Eocene Climatic Optimum, suggesting inhibitive equatorial temperatures during the peak of the Cenozoic hothouse; increased specialization due to circulation changes across the Eocene-Oligocene transition, preceding the loss of morphological diversity; changes in morphological specialization and richness about 19 million years ago, coeval with pelagic shark extinctions5; delayed onset of changing functional group richness and specialization between hemispheres during the mid-Miocene plankton diversification. The detailed nature of the Triton dataset permits a unique spatiotemporal view of Cenozoic pelagic macroevolution, in which global biogeographic responses of functional communities and richness are decoupled during Cenozoic climate events. The global response of functional groups to similar abiotic selection pressures may depend on the background climatic state (greenhouse or icehouse) to which a group is adapted.

Late Cenozoic cooling restructured global marine plankton communities

The geographic ranges of marine organisms, including planktonic foraminifera¹, diatoms, dinoflagellates², copepods³ and fish⁴, are shifting polewards owing to anthropogenic climate change⁵. However, the extent to which species will move and whether these poleward range shifts represent precursor signals that lead to extinction is unclear⁶. Understanding the development of marine biodiversity patterns over geological time and the factors that influence them are key to contextualizing these current trends. The fossil record of the macroperforate planktonic foraminifera provides a rich and phylogenetically resolved dataset that provides unique opportunities for understanding marine biogeography dynamics and how species distributions have responded to ancient climate changes. Here we apply a bipartite network approach to quantify group diversity, latitudinal specialization and latitudinal equitability for planktonic foraminifera over the past eight million years using Triton, a recently developed high-resolution global dataset of planktonic foraminiferal occurrences⁷. The results depict a global, clade-wide shift towards the Equator in ecological and morphological community equitability over the past eight million years in response to temperature changes during the late Cenozoic bipolar ice sheet formation. Collectively, the Triton data indicate the presence of a latitudinal equitability gradient among planktonic foraminiferal functional groups which is coupled to the latitudinal biodiversity gradient only through the geologically recent past (the past two million years). Before this time, latitudinal equitability gradients indicate that higher latitudes promoted community equitability across ecological and morphological groups. Observed range shifts among marine planktonic microorganisms^1,2,8 in the recent and geological past suggest substantial poleward expansion of marine communities even under the most conservative future global warming scenarios.

Plankton response to global warming is characterized by non-uniform shifts in assemblage composition since the last ice age

Biodiversity is expected to change in response to future global warming. However, it is difficult to predict how species will track the ongoing climate change. Here we use the fossil record of planktonic foraminifera to assess how biodiversity responded to climate change with a magnitude comparable to future anthropogenic warming. We compiled time series of planktonic foraminifera assemblages, covering the time from the last ice age across the deglaciation to the current warm period. Planktonic foraminifera assemblages shifted immediately when temperature began to rise at the end of the last ice age and continued to change until approximately 5,000 years ago, even though global temperature remained relatively stable during the last 11,000 years. The biotic response was largest in the mid latitudes and dominated by range expansion, which resulted in the emergence of new assemblages without analogues in the glacial ocean. Our results indicate that the plankton response to global warming was spatially heterogeneous and did not track temperature change uniformly over the past 24,000 years. Climate change led to the establishment of new assemblages and possibly new ecological interactions, which suggests that current anthropogenic warming may lead to new, different plankton community composition.

Horizontal Distribution and Carbon Biomass of Planktonic Foraminifera in the Eastern Indian Ocean

Distribution and carbon biomass of planktonic foraminifera were investigated from the euphotic zone of the Eastern Indian Ocean during a two-month cruise, ‘Shiyan I’ (10 April–13 May 2014). Foraminifera species were collected through plankton net sampling at 44 locations (80.00°–96.10° E, 10.08° N–6.00° S). The temperature (°C) ranged between 12.82 and 31.8 °C, the salinity ranged between 32.5 and 35.5, and chlorophyll-a concentrations ranged between 0.005 µg/L and 0.89 µg/L. A total of 20 taxa were identified based on the spherical chamber shell, spines, and a final whorl which were examined under light microscopy and scanning electron microscopy. Dominant species that were characterized by the high dominant index Y > 0.14–0.46 were Globigerina bulloides, Globigerinoides ruber white, Globigerinella siphonifera, Turborotalita quinqueloba, and Globigerinella calida, contributing to the community up to 86%. The shell size of collected taxa was from 51 to 508 μm and the total carbon biomass was estimated to be between 0.062 µg C m–3 and 26.52 µg C m–3. The high carbon biomass was recorded at two stations in the equator zone. Due to its large size, Globorotalia menardii had total carbon biomass of 3.9 µg C m–3, followed by G. calida 0.68 µg C m−3, Trilobatus sacculifer 0.38 µg C m–3, Orbulina universa 0.56 µg C m–3, and G. ruber white 0.22 µg C m–3, respectively. The Pearson correlation analysis showed that the temperature and chlorophyll-a were two explanatory environmental variables that were found to be highly significant (p < 0.05) and that triggered the distribution and abundance of dominant foraminifera species in the study region. Overall, high abundances and carbon biomass were derived from the euphotic zone and equatorial region of the Eastern Indian Ocean.

Diachroneity Rules the Mid-Latitudes: A Test Case Using Late Neogene Planktic Foraminifera across the Western Pacific

Planktic foraminifera are commonly used for first-order age control in deep-sea sediments from low-latitude regions based on a robust tropical–subtropical zonation scheme. Although multiple Neogene planktic foraminiferal biostratigraphic zonations for mid-latitude regions exist, quantification of diachroneity for the species used as datums to test paleobiogeographic patterns of origination and dispersal is lacking. Here, we update the age models for seven southwest-Pacific deep-sea sites using calcareous nannofossil and bolboform biostratigraphy and magnetostratigraphy, and use 11 sites between 37.9° N and 40.6° S in the western Pacific to correlate existing planktic foraminiferal biozonations and quantify the diachroneity of species used as datums. For the first time, northwest and southwest Pacific biozones are correlated and compared to the global tropical planktic foraminiferal biozonation. We find a high degree of diachroneity in the western Pacific, within and between the northwest and southwest regions, and between the western Pacific and the tropical zonation. Importantly, some datums that are found to be diachronous between regions have reduced diachroneity within regions. Much work remains to refine regional planktic foraminiferal biozonations and more fully understand diachroneity between the tropics and mid-latitudes. This study indicates that diachroneity is the rule for Late Neogene planktic foraminifera, rather than the exception, in mid-latitude regions.

Decadal trend of plankton community change and habitat shoaling in the Arctic gateway recorded by planktonic foraminifera

The Fram Strait plays a crucial role in regulating the heat and sea-ice dynamics in the Arctic. In response to the ongoing global warming, the marine biota of this Arctic gateway is experiencing significant changes with increasing advection of Atlantic species. The footprint of this 'Atlantification' has been identified in isolated observations across the plankton community, but a systematic, multi-decadal perspective on how regional climate change facilitates the invasion of Atlantic species and affects the ecology of the resident species is lacking. Here we evaluate a series of 51 depth-resolved plankton profiles collected in the Fram Strait during seven surveys between 1985 and 2015, using planktonic foraminifera as a proxy for changes in both the pelagic community composition and species vertical habitat depth. The time series reveals a progressive shift towards more Atlantic species, occurring independently of changes in local environmental conditions. We conclude that this trend is reflecting higher production of the Atlantic species in the Nordic Seas, from where they are advected into the Fram Strait. At the same time, we observe the ongoing extensive sea-ice export from the Arctic and associated cooling-induced decline in density and habitat shoaling of the subpolar Turborotalita quinqueloba, whereas the resident Neogloboquadrina pachyderma persists. As a result, the planktonic foraminiferal community and vertical structure in the Fram Strait shift to a new state, driven by both remote forcing of the Atlantic invaders and local climatic changes acting on the resident species. The strong summer export of Arctic sea ice has so far buffered larger plankton transformation. We predict that if the sea-ice export will decrease, the Arctic gateway will experience rapid restructuring of the pelagic community, even in the absence of further warming. Such a large change in the gateway region will likely propagate into the Arctic proper.

Thermal niches of planktonic foraminifera are static throughout glacial-interglacial climate change

Abiotic niche lability reduces extinction risk by allowing species to adapt to changing environmental conditions in situ. In contrast, species with static niches must keep pace with the velocity of climate change as they track suitable habitat. The rate and frequency of niche lability have been studied on human timescales (months to decades) and geological timescales (millions of years), but lability on intermediate timescales (millennia) remains largely uninvestigated. Here, we quantified abiotic niche lability at 8-ka resolution across the last 700 ka of glacial-interglacial climate fluctuations, using the exceptionally well-known fossil record of planktonic foraminifera coupled with Atmosphere-Ocean Global Climate Model reconstructions of paleoclimate. We tracked foraminiferal niches through time along the univariate axis of mean annual temperature, measured both at the sea surface and at species' depth habitats. Species' temperature preferences were uncoupled from the global temperature regime, undermining a hypothesis of local adaptation to changing environmental conditions. Furthermore, intraspecific niches were equally similar through time, regardless of climate change magnitude on short timescales (8 ka) and across contrasts of glacial and interglacial extremes. Evolutionary trait models fitted to time series of occupied temperature values supported widespread niche stasis above randomly wandering or directional change. Ecotype explained little variation in species-level differences in niche lability after accounting for evolutionary relatedness. Together, these results suggest that warming and ocean acidification over the next hundreds to thousands of years could redistribute and reduce populations of foraminifera and other calcifying plankton, which are primary components of marine food webs and biogeochemical cycles.

Subtropical to temperate late Neogene to Quaternary planktic foraminiferal biostratigraphy across the Kuroshio Current Extension, Shatsky Rise, northwest Pacific Ocean

Planktic foraminiferal biostratigraphic zonation schemes are critical for providing first-order relative age control in deep-sea sediments and provide the basis on which to interpret evolutionary dynamics through time. Over the previous decades, the majority of published biostratigraphic zonation schemes focused on the tropical regions of the world. The mid-latitude or temperate regions, especially of the northwest Pacific, have been understudied in terms of recording plankton occurrences. Lack of detailed biostratigraphic studies have largely left out this region from plankton evolutionary analyses, thus how this part of the world ocean, which is characterized by the Kuroshio Current Extension (KCE), may contribute to global plankton biodiversity is unknown. In this study, we present the first magnetostratigraphically-calibrated late Neogene to Quaternary (15.12-0 Ma) planktic foraminiferal zonation schemes from the northwest Pacific for three Ocean Drilling Program Leg 198 holes (1207A, 1208A, and 1209A) that span the KCE. We utilize previously published warm subtropical, cool subtropical, and temperate zonation schemes from the southwest Pacific, with modifications. We find examples of significant diachroneity among primary marker taxa used to construct biozones at the three northwest Pacific sites, which ranges from 0.075 to 2.29 million years. Comparison of our primary datum markers with those of the tropical planktic foraminiferal zonation scheme also reveal diachroneity on the scale of 0.022 to 4.8 million years. We have identified times of intense dissolution in the northwest Pacific, namely in the middle to late Miocene that likely contribute to the observed diachroneity of datums. This study highlights the need for regionally specific mid-latitude biostratigraphic zonation schemes, as diachronous datums and differing assemblages may be hallmarks of oceanic ecotones created by major boundary current systems. These data also provide a framework to characterize local plankton evolutionary dynamics and paleobiogeographic patterns in future studies.

Past and future decline of tropical pelagic biodiversity

A major research question concerning global pelagic biodiversity remains unanswered: when did the apparent tropical biodiversity depression (i.e., bimodality of latitudinal diversity gradient [LDG]) begin? The bimodal LDG may be a consequence of recent ocean warming or of deep-time evolutionary speciation and extinction processes. Using rich fossil datasets of planktonic foraminifers, we show here that a unimodal (or only weakly bimodal) diversity gradient, with a plateau in the tropics, occurred during the last ice age and has since then developed into a bimodal gradient through species distribution shifts driven by postglacial ocean warming. The bimodal LDG likely emerged before the Anthropocene and industrialization, and perhaps ∼15,000 y ago, indicating a strong environmental control of tropical diversity even before the start of anthropogenic warming. However, our model projections suggest that future anthropogenic warming further diminishes tropical pelagic diversity to a level not seen in millions of years.

Temperature is a poor proxy for synergistic climate forcing of plankton evolution

Changes in biodiversity at all levels from molecules to ecosystems are often linked to climate change, which is widely represented univariately by temperature. A global environmental driving mechanism of biodiversity dynamics is thus implied by the strong correlation between temperature proxies and diversity patterns in a wide variety of fauna and flora. Yet climate consists of many interacting variables. Species probably respond to the entire climate system as opposed to its individual facets. Here, we examine ecological and morphological traits of 12 633 individuals of two species of planktonic foraminifera with similar ecologies but contrasting evolutionary outcomes. Our results show that morphological and ecological changes are correlated to the interactions between multiple environmental factors. Models including interactions between climate variables explain at least twice as much variation in size, shape and abundance changes as models assuming that climate parameters operate independently. No dominant climatic driver can be identified: temperature alone explains remarkably little variation through our highly resolved temporal sequences, implying that a multivariate approach is required to understand evolutionary response to abiotic forcing. Our results caution against the use of a ‘silver bullet’ environmental parameter to represent global climate while studying evolutionary responses to abiotic change, and show that more comprehensive reconstruction of palaeobiological dynamics requires multiple biotic and abiotic dimensions.

The impact of Cenozoic cooling on assemblage diversity in planktonic foraminifera

The Cenozoic planktonic foraminifera (PF) (calcareous zooplankton) have arguably the most detailed fossil record of any group. The quality of this record allows models of environmental controls on macroecology, developed for Recent assemblages, to be tested on intervals with profoundly different climatic conditions. These analyses shed light on the role of long-term global cooling in establishing the modern latitudinal diversity gradient (LDG)--one of the most powerful generalizations in biogeography and macroecology. Here, we test the transferability of environment-diversity models developed for modern PF assemblages to the Eocene epoch (approx. 56-34 Ma), a time of pronounced global warmth. Environmental variables from global climate models are combined with Recent environment-diversity models to predict Eocene richness gradients, which are then compared with observed patterns. The results indicate the modern LDG--lower richness towards the poles--developed through the Eocene. Three possible causes are suggested for the mismatch between statistical model predictions and data in the Early Eocene: the environmental estimates are inaccurate, the statistical model misses a relevant variable, or the intercorrelations among facets of diversity--e.g. richness, evenness, functional diversity--have changed over geological time. By the Late Eocene, environment-diversity relationships were much more similar to those found today.